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Not enough parking, you say? A study of garage use and parking supply for
single-family homes in Sacramento and implications for ADUs
Article in Journal of Transport and Land Use · February 2022
DOI: 10.5198/jtlu.2022.1947
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1 Introduction
Accessory dwelling units—or “ADUs”—are smaller, self-contained housing units that share the same
lot as a larger primary dwelling, usually a single-family detached house (Chapple et al., 2017).1 ADUs
can be constructed anew (either as standalone structures or extensions of the primary dwelling) or
converted from existing structures on the lot (like garages) or an interior portion of the larger dwelling.
ADUs are often touted as a time- and cost-effective way to increase housing supply, particularly in areas
like California with major housing supply shortages (Casey, 2020; Woetzel et al., 2016). One study
even estimated that ADU-like developments could provide half of California’s new housing capacity in
Article history:
Received: December 18, 2020
Received in revised form: August
14, 2021
Accepted: December 9, 2021
Available online: February 28,
2022
Copyright 2022 Jamey M. B. Volker & Calvin G. Thigpen
http://dx.doi.org/10.5198/jtlu.2022.1947
ISSN: 1938-7849 | Licensed under the Creative Commons Attribution – Noncommercial License 4.0
The Journal of Transport and Land Use is the official journal of the World Society for Transport and Land Use (WSTLUR)
and is published and sponsored by the University of Minnesota Center for Transportation Studies. This paper is also
published with additional sponsorship from WSTLUR.
1 It was not until 2019 that California state law was amended to explicitly allow ADUs on lots with multifamily dwellings
(California Assembly Bill 881, 2019-2020).
T J T L U http://jtlu.org
V. 15 N. 1 [2022] pp. 183–206
Jamey M. B. Volker (corresponding author)
Institute of Transportation Studies
University of California, Davis
jvolker@ucdavis.edu
Calvin G. Thigpen
thigpen.calvin.g@gmail.com
Not enough parking, you say? A study of garage use and parking
supply for single-family homes in Sacramento and implications
for ADUs
Abstract: Accessory dwelling units (ADUs) are increasingly touted as
part of the solution to the intransigent housing shortages facing many
metropolitan areas across the United States. But numerous barriers
to ADU development persist, including opposition by neighboring
households. One persistent question is whether ADU residents would
overwhelm on-street parking in the predominately single-family
neighborhoods where ADUs are typically built. That question is difficult
to answer because there is a surprising dearth of research on the effective
parking supply in single-family neighborhoods. We use a survey of
homeowners in Sacramento, California, to investigate the supply and
sufficiency of residential parking for single-family homes, including how
households actually use their garages, and help answer the ADU parking
conundrum. After estimating and accounting for actual garage use, we
find that more than 75% of households have enough off-street parking
available to park all their vehicles. When we combine off-street and on-
street parking supplies, we find that households have an average of 1.6
more parking spaces available to them than they have vehicles. That
parking surplus is more than enough to accommodate the average ADU
tenant and their vehicle, belying claims that ADUs will overwhelm
existing parking supplies in single-family neighborhoods.
184 JOURNAL OF TRANSPORT AND LAND USE 15.1
the coming decades (Woetzel et al., 2016). But spurring ADU development has proved difficult.
In addition to difficulties obtaining financing and other obstacles, homeowners across the United
States have faced significant regulatory barriers to permitting ADUs. One of the most significant barri-
ers has been onerous parking mandates, like requiring provision of additional off-street parking for the
ADU or requiring the replacement of covered parking spaces that are eliminated to make space for the
ADU (Anacker & Niedt, 2019; Brown et al., 2017; Chapple et al., 2012; Cho, 2016; Morales, 2019;
Pfeiffer, 2019; Wegmann & Chapple, 2012). In an effort to reduce these parking-related burdens and
other regulatory barriers, California recently changed state law to make it easier than ever before to
permit ADUs (California Government Code Section 65852.2, n.d.; Volker, 2020). Some local gov-
ernments have loosened the regulations even further. In Sacramento—the geographical focus of this
study—the city does not require off-street parking spaces for ADUs under any circumstance (City of
Sacramento, 2021a).
Perversely, however, reducing off-street parking requirements can exacerbate neighborhood op-
position to ADUs. Insufficient on-street parking is already one of the concerns most commonly cited
by neighboring households in opposing proposed ADUs (Anacker & Niedt, 2019; Brown et al., 2017;
Chapple et al., 2012; Wegmann & Chapple, 2012). And ADUs are more likely to increase demand for
on-street parking if there is no requirement that homeowners add off-street parking to accommodate
the ADU-generated vehicles or replace any off-street parking spaces eliminated to make room for the
ADU. Indeed, the results from a recent survey of California’s local governments indicate that “[w]ith the
new state ADU legislation, . . . the public is the most concerned about parking” (Chapple et al., 2020).
That begs the question—is the total effective parking supply of the average single-family detached home
sufficient to accommodate the vehicles generated by both the household in the primary dwelling and
the household in a potential ADU?
This seemingly simple question of supply and demand is not easily answered with existing studies.
One might expect there to be ample evidence on parking supply and demand in single-family neigh-
borhoods, given the prevalence of minimum residential parking standards across the US. Despite their
importance, however, those standards are notoriously based on thin to non-existent evidence (Guo et
al., 2012; Willson, 2000). We likewise found a dearth of research on the effective parking supply in
single-family neighborhoods, let alone research that compares supply to actual demand. This study
helps fill those knowledge gaps and answer the ADU parking conundrum using a survey of single-family
detached homeowners in Sacramento, California (n = 396).
In the rest of the paper, we review the literature on parking supply in single-family neighborhoods,
survey the research on garage use (in order to ascertain actual availability for parking), and describe how
we evaluated garage use and estimated effective parking supply and surplus. We then compare our find-
ings to previous studies and discuss the implications for ADU development. Considering total effective
parking supply (off-street and on-street combined), we find that the average household has 1.6 to 2.0
more parking spaces available to them than they have vehicles, suggesting a sufficient parking supply to
accommodate ADUs in single-family home neighborhoods.
2 Literature review
The total or “nominal” parking supply for a single-family detached home equals (1) the number of
on-street parking spaces generally available to that household plus (2) the number of off-street parking
spaces nominally available to the household for parking a vehicle. The “effective” parking supply for a
single-family detached home equals the total parking supply minus the number of off-street spaces actu-
ally used for other purposes. The effective parking “surplus” equals the effective parking supply minus
185Not enough parking, you say? A study of garage use and parking supply for single-family homes in Sacramento
the number of vehicles parked on site by the occupants of the parcel (be it homeowners, ADU renters, or
others). We first review the studies that attempt to look at the big picture—total parking supply, effective
parking supply, and/or effective parking surplus. We then review the related but distinct subset of studies
that focus on the actual use of off-street parking (primarily garages).
2.1 Parking supply and surplus for single-family detached homes
Single-family detached homes often have both private off-street parking (like garages, carports, drive-
ways, or even yards) and access to on-street parking (Guo, 2013). Despite frequent access to on-street
parking, local zoning laws in many US cities require that single-family homes contain off-street parking
for the purpose of car storage (Guo et al., 2012). Data for both on- and off-street parking are needed to
estimate the total effective parking supply and surplus for a given household or in a given neighborhood.
But this data is often lacking or unavailable (Guo, 2013; Guo & Schloeter, 2013; Taylor, 2020; Thigpen
& Volker, 2017). Consequently, the literature contains scant research on parking supplies and surpluses
in single-family neighborhoods, as previous reviews confirm (Anirudh et al., 2021; Guo, 2013; Scheiner
et al., 2020; Taylor, 2020; Thigpen & Volker, 2017).
Guo et al. (2012) and Guo ( 2013) come the closest we have seen to estimating a total parking sup-
ply for single-family homes. Guo et al. ( 2012) investigated on-street parking requirements in residential
neighborhoods in the US. As part of their analysis, they estimated that the average single-family home
in the US has access to 8-10 parking spaces, including 2.6 garage spaces, 4-5 on-street spaces, and 1-3
driveway spaces. They calculated the number of garage spaces using data on garage size (i.e., theoretical
garage capacity, not actual availability for auto parking after accounting for other garage uses). They
estimated the number of on-street spaces using an average lot size (0.34 acres) for single-family homes
built in US metro areas between 1970 and 2010, an assumed frontage-depth ratio of 2:3 for those lots,
and an average length of 20 feet for on-street parking spaces (Guo et al., 2012). They estimated an aver-
age of 110 feet of street line per home, which translates to four or five parking spaces, depending on the
curb cut length. Guo (2013) replicated the same calculus in a subsequent article analyzing the effect of
on-street parking on car ownership in the New York City region.
Thigpen and Volker (2017), Schlossberg and Amos (2015), and Roth (2016) calculated on-street
parking supply and vacancy rates for predominately single-family neighborhoods in Davis, California,
Eugene, Oregon, and Bellevue, Washington, respectively. They all found that on-street parking was
oversupplied, with vacancy rates between 71% and 89% at times of peak occupancy. Schlossberg and
Amos (2015) also tallied driveway parking spaces in the same Eugene neighborhoods. But none of the
three studies estimated total parking supplies or surplus for the households in the subject neighbor-
hoods, let alone extrapolated the results beyond their specific samples.
As part of a study on the market for ADUs, Wegmann and Chapple (2012) calculated the average
number of available off-street spaces for a (nearly) random sample of single-family homeowners near
Bay Area Rapid Transit stations in five cities on the east side of the San Francisco Bay Area. They found
that households with at least one ADU reported an average of 1.86 available off-street parking spaces
(in garages, driveways, or elsewhere off street), while households without ADUs reported an average of
1.67 available off-street spaces (an insignificant difference statistically). They also calculated the average
number of “overspill” vehicles—the difference between the number of cars generated by the property
(including any ADUs) and the number of available off-street parking spaces. They found that properties
with ADUs had an average overspill of 1.09 vehicles, while households without ADUs had an average
overspill of 0.35 vehicles. However, they did not estimate the number of available on-street spaces per
home or, consequently, total effective parking surplus.
Taylor (2020) presented more comprehensive household survey data than previous studies about
186 JOURNAL OF TRANSPORT AND LAND USE 15.1
residential parking availability, use, and sufficiency for a sample of households in Melbourne, Australia,
59% of whom lived in a single-family detached home. Taylor found that 74% of respondents had “suf-
ficient” off-street parking, i.e., they had at least the same number of available off-street spaces as vehicles.
But the study did not present the total effective parking supply or surplus for those households.
Overall, the available literature indicates that parking is generally oversupplied in single-family
neighborhoods. But estimates of the total effective parking supply and surplus for single-family de-
tached homes are sorely lacking, particularly estimates using actual—or “effective”—off-street parking
space availability (adjusting the nominal off-street capacity based on how households actually use those
spaces). Our study helps fill this gap.
2.2 Use of off-street parking
Estimating effective parking supply and surplus requires knowing how ostensibly available off-street
parking spaces are actually used, particularly garage parking (since it is enclosed and therefore more
likely to be used for purposes other than parking). Most studies of parking supply in single-family neigh-
borhoods do not address this in detail, as discussed above. The few studies that do investigate off-street
parking use focus on garages.
Many of the contemporary studies of personal garage use in the US focus on the Los Angeles
region. In 1987, Los Angeles Times reporters undertook a systematic study of garage use and found that
3.2% were inhabited (Chavez & Quinn, 1987). A subsequent study of 32 “middle-class” households
reported that 75% of garages were used exclusively for storage of household belongings (Arnold et al.,
2012). The authors further estimated that roughly 90% of garage space in middle-class Los Angeles
neighborhoods is used for storage rather than parking. A peer-reviewed journal article using a subset of
the same data provided additional detail (Arnold & Lang, 2007). According to the authors, only one
household out of twenty-four did not use some or all of their garage for storage. The study’s methodolo-
gy, based on archaeological approaches, notes that garages are overwhelming used by this segment of the
population as an overflow area for material possessions and food. More informally, a City of Compton
official estimated that roughly a third of the city’s garages were used for housing people rather than cars
(Garrison, 2009). And an informal estimate by Wegmann (2015) suggested that a majority of garages
in the Southeast Los Angeles area had been converted for residential purposes.
Beyond the Los Angeles region, we identified seven other studies that attempted to estimate garage
uses. In a survey of planning professionals in nearly 100 major US cities, over a quarter identified storage
as a “very common” use for garages and another two-thirds said it was “common” (Guo & Schloeter,
2013). A study of garages in the Mission District neighborhood of San Francisco found that 49% of the
97 garages observed during field observations were not used for car parking (Brown, 2007). In a 1998
New York Metropolitan Transportation Council survey, only 13% of households with garages parked
their car in the garage - the remainder parked cars in their driveway and on the street (Guo, 2013).
Jenks and Noble (1996) surveyed homeowners with one-car garages in a suburb outside of Reading,
England and found that 38% of garages were not used to store vehicles (as cited in Guo & Schloeter,
2013, 460). A review of studies conducted in the United Kingdom found that between 36% and 45%
of garages were used for car parking (United Kingdom Department for Transport, 2007). More recently,
an opinion panel of UK drivers similarly found that 53% of drivers with garages never use them for car
parking and only 40% park in their garage on a regular basis (RAC Media Center, 2021). In a survey
of Melbourne, Australia residents, Taylor (2020) found that of the respondents living in single-family
detached homes, 41% used their off-street parking only for car parking, 35% for car parking and other
uses (e.g., storage, living areas), and 18% for only non-car parking purposes.
187Not enough parking, you say? A study of garage use and parking supply for single-family homes in Sacramento
Table 1. Estimates of garage uses in the literature
Study Setting Methods Living/ADU Storage Parking
Arnold et al.
(2012)
Los Angeles area Ethnographic
study of 32 dual-
earner households
-75%-
Brown (2007)San Francisco Field observation --51%
Chavez & Quinn
(1987)
Los Angeles
County
Systematic sam-
pling
3.2%--
United Kingdom
Department for
Transport (2007)
England Review of the
literature
--36-45%
Garrison (2009)Compton,
California
Informal estimate 33%--
Guo et al. (2012)United States Expert opinion
survey
-27%: “very
common;” 63%:
“common”
-
Guo (2013)New York City Survey of
households
--13%a
Jenks & Noble
(1996), as cited in
Guo and Schlo-
eter (2013, 460)
United Kingdom
suburb
Survey of one-car
garage households
--62%
RAC (2021)United Kingdom Opinion panel
survey
40% regularly;
8% occasionally
Taylor (2020)Melbourne,
Australia
Convenience
sampling
-18%41% only, 35%
with storage or
other use
Wegmann (2015)Southeast
Los Angeles
Informal estimate majority --
a Statistic refers to houses with garages
Other studies have also weighed in on the topic of garage use or, more expansively, off-street park-
ing use. For example, a study of residential parking patterns in a dense, mixed-use neighborhood of
Dortmund, Germany found that roughly a third (37%) of households with private off-street parking al-
ways used that space to store a car, while ten percent never used it for car parking (Scheiner et al., 2020).
Our study adds to this small, yet slowly growing, body of research on off-street parking use.
3 Methods
3.1 Setting
We conducted this study in Sacramento—California’s capital city and its sixth most populous, with
over 508,000 residents (US Census Bureau, 2018a). California is a prime location to study residential
parking supply and its implications for ADU development. The state is mired in a major housing supply
and affordability crisis that has been exacerbated by decades of planning and building for automobility,
including capacious street width standards and off-street parking requirements (Guo et al., 2012; Volker
et al., 2019). The state government has recently embraced ADUs as an integral part of solving the hous-
ing crisis and changed state law to make it easier than ever before to permit ADUs (California Govern-
188 JOURNAL OF TRANSPORT AND LAND USE 15.1
ment Code Section 65852.2, n.d.; Volker, 2020). Yet concerns about inadequate parking in residential
neighborhoods—especially single-family neighborhoods—could hinder ADU construction.
Sacramento is a natural setting for this study within California because single-family detached homes
dominate the city’s housing stock to a greater extent than most of the state’s other urban areas. Over
60.4% of Sacramento’s housing units are single-family detached houses (US Census Bureau, 2018b). In
total, Sacramento has over 118,000 single-family detached homes (US Census Bureau, 2018b).
Sacramento is also a prime location for studying the implications of residential parking supply for
housing development, and particularly ADUs, because it is one of California’s fastest growing cities. Sac-
ramento added more people between 2018 and 2019 than all but one (Chico) of California’s 482 cities
(California Department of Finance, 2019). And it grew at a faster rate (1.49%) than any of the other
top-10 most populous cities in the state (California Department of Finance, 2019). Yet the city actually
lost 2.4% of its housing units between 2017 and 2018 (US Census Bureau, 2018a, 2017). Meanwhile,
Sacramento’s vacancy rate dropped from 6.6% in 2010 to 4.2% in 2018 (US Census Bureau, 2018b,
2010).
The conditions would appear particularly ripe for ADU development in Sacramento because up to
47% of single-family detached homeowners in Sacramento could be open to building an ADU accord-
ing to a recent study that used the same survey data we use in this study (Volker & Handy, in press).
Yet the numbers of ADU permits applied for and issued in Sacramento remain relatively low compared
to the jump in ADU permitting seen in other large cities following California’s relaxation of ADU re-
strictions starting in 2016. Sacramento received only 162 ADU permit applications in 2020 (City of
Sacramento, 2021b), up from 83 in 2019.2
Parking-related impediments are certainly not the only reason why Sacramento has lagged behind
other large California cities in ADU permitting. Indeed, Sacramento does not require off-street parking
for ADUs (City of Sacramento, 2021a). Nor, per state law, does it require the replacement of covered
off-street parking spaces eliminated to make room for an ADU (California Government Code Section
65852.2, n.d.). But neighborhood opposition fueled by concerns about insufficient parking and traffic
can be a formidable obstacle for homeowners who are considering adding an ADU. Another analysis of
the same 2019 survey data that we use in this study found that the majority of Sacramento-area single-
family homeowners who reported being open to adding an ADU ranked “opposition from neighbors”
as at least a “minor obstacle” to creating an ADU (Volker, 2020). And parking and traffic issues appeared
to be the most likely source of opposition. When asked whether they thought that a neighbor renting
out an ADU would “have a negative impact on the neighborhood,” 46% of all surveyed homeowners
responded that it would result in “too many cars trying to park on my street.”
3.2 Gauging garage use and effective off-street parking supply
To better understand garage use and effective off-street parking supply in single-family detached homes,
we surveyed a sample of single-family detached homeowners in Sacramento in the summer of 2019
(July through August).
3.2.1 Data collection and respondent snapshot
We recruited participants for our online survey from three separate sources. Our first recruitment came
from a set of double-opt-in panels maintained by the experience management company Qualtrics.
Qualtrics administered the survey in late July and early August 2019 to 351 respondents who passed
the screening question, finished the online survey, and did not provide any unintelligible or clearly
2 Matt Hertel, Principal Planner, City of Sacramento, email message to author, April 15, 2020.
189Not enough parking, you say? A study of garage use and parking supply for single-family homes in Sacramento
unresponsive answers to the text response questions (to weed out unserious respondents). We obtained
our second recruitment list through the 2018-2019 campus travel survey at the University of Califor-
nia, Davis; 143 respondents said they would be willing to let us contact them again for this study. We
garnered the third recruitment list through two surveys about the JUMP bikeshare program in the
Sacramento region; 440 respondents replied that they would be willing to take a future survey. We
compared the recruitment lists from the campus travel survey and JUMP bike surveys and removed any
duplicate email addresses. We emailed an invitation and link to take our ADU survey to all 583 unique
recruits from the second and third recruitment lists in mid-August 2019. Over half (312) of the recruits
both passed the screening question (owning their own home) and at least partially completed the online
survey. None reported having previously taken our survey as part of a Qualtrics panel, but we could
not independently verify this because the names and email addresses for the Qualtrics respondents were
kept confidential. As an additional check for “ballot-box stuffing”, we examined the socio-demographic
responses and did not identify any duplicates, providing supportive evidence that responses came from
unique respondents. In total, we garnered 663 unique respondents across the three recruitment pools.
To further improve data quality, we excised the 106 respondents who either did not answer the pri-
mary questions of interest or failed to answer at least five other questions. We then culled the remaining
557 complete cases by removing (1) all respondents who did not own a single-family detached home,
and (2) all respondents who lived outside of the City of Sacramento. That yielded a final sample size of
396 single-family detached homeowners living in Sacramento.
Table 2 compares the housing stock and demographic characteristics of our sample to those of the
population of interest—householders owning a single-family detached home in Sacramento. Our sam-
ple is younger, whiter, more educated, and has a greater percentage of people who have lived in the same
home for less than 10 years than the homeowner population of Sacramento. The average household size
and percentage of households with more than two vehicles are also slightly greater in our sample than in
the city’s overall population of homeowning householders.
Table 2. Characteristics of sampled homeowners compared to the population of Sacramento homeowners
City of Sacramento
Homeowner Sample
City of Sacramento Home-
owner Populationª
Sample size 396 -
Housing Stockª
Number of owner-occupied housing units 396 89,678 (+/- 4,030)
Number of owner-occupied single-family detached units 396 79,816 (+/- 3,770)
Demographicsa
Median age or age bracket 46 (45-54)55-64
Median income or income bracket $75,000 - $99,999 $82,959 (+/- 1,525)
Share with bachelor’s degree or higher 63.1%45.4% (+/- 1.0)
Share non-Hispanic white 60.4%48.9% (+/- 0.9)
Share with 10 or fewer years in current homeb 49.5%33.6%
Share with 3 or more vehicles 24.8%24.0%
Average household size 2.87 2.76 (+/- 0.03)
Notes:
a Housing stock estimates for the City of Sacramento obtained from table S2504 from the American Community Survey’s
2018 1-year estimates (US Census Bureau, 2018a). Demographic estimates obtained from tables S2502, S2503, B25010,
and B25044 from the American Community Survey’s 2018 5-year estimates (US Census Bureau, 2018b). Demographic
estimates are for all owner-occupied housing units, the majority of which are single-family detached units.
b For the City of Sacramento homeowner population, this equals the share of homeowners who have moved into their cur-
rent home in 2010 or later (8 or fewer years before the 2018 American Community Survey).
190 JOURNAL OF TRANSPORT AND LAND USE 15.1
3.2.2 Survey weighting
We weighted the sample to represent the population of single-family homeowners in Sacramento using
two key variables likely to influence garage and off-street parking use: household size and number of cars
available. In order to estimate the full cross-tabulation of these two characteristics, for the population of
owner-occupied, single-family detached households in the City of Sacramento, we downloaded 2019
5-year American Community Survey Public Use Microdata Sample (PUMS) data from the Census for
the Public Use Microdata Areas (PUMAs) that contain the City of Sacramento boundaries (US Census
Bureau, 2003a, 2003b). By using PUMS data, we were able to estimate cell values for the contingency
table of household size by vehicle availability and therefore use post-stratification cell weighting (Kalton
& Flores-Cervantes, 2003). PUMAs align closely, but not perfectly, with City of Sacramento boundar-
ies. We tested the sensitivity of the contingency table’s cell values to which PUMAs were included to
represent our population and found minimal differences - the greatest difference between any two sets
of PUMA combinations was two percentage points for one cell.
Because the cell sizes for households with 0 cars and 4 or more cars in our sample were very small,
we collapsed those categories together with the adjacent vehicle availability categories, yielding 3 vehicle
availability categories for weighting: 1 or less, 2, and 3 or more cars. The weights range from a low of
0.51 to a high of 3.55 (Table 3).
Table 3. Survey weights for sample based on household size and cars available
Household size Cars available Sample Census Percentage point
difference
(Sample - Census)
Survey weight
1
1 or less 7.1%6.1%0.9 0.86
2 1.8%1.5%0.3 0.86
3 or more 0.8%0.4%0.3 0.67
2
1 or less 6.8%5.3%1.5 0.78
2 26.5%14.5%12.0 0.54
3 or more 8.8%4.6%4.3 0.51
3
1 or less 2%2.5%-0.5 1.25
2 11.9%8%3.8 0.68
3 or more 5.3%6.6%-1.3 1.24
4
1 or less 1.5%1.5%0.0 1.00
2 12.9%10.5%2.4 0.82
3 or more 4.8%8.8%-4.0 1.84
5 or more
1 or less 1.5%2.3%-0.8 1.50
2 3.3%9.4%-6.1 2.85
3 or more 5.1%18%-12.9 3.55
3.2.3 Survey questions and data analysis
We used our weighted data to explore off-street parking supply and surplus, including garage use. Table
4 shows the relevant questions we asked respondents about garage availability, garage use, the availability
of other off-street parking, and vehicle availability.
191Not enough parking, you say? A study of garage use and parking supply for single-family homes in Sacramento
Table 4. Survey questions used in garage use and off-street parking supply analyses
Questions Answer options
Does your home have a garage?
Yes, a 1-car garage
Yes, a 2-car garage
Yes, a 3+-car garage
No
How do you use your garage? Please check all that apply.
For automobile storage and repair
For other storage
As an additional bedroom or independent living space for
my household
As an independent living space rented to others
As an office, play room, laundry room, or other type of
additional room
Other. Please specify:
How many off-street automobile parking spaces does your
home have outside of a garage (e.g. spaces in an uncovered
driveway or covered carport)?
Numeric response
How many automobiles does your household own (or lease)
and park at home?
Numeric response
For our garage use analysis, we used the weighted responses to the “How do you use your garage?”
question shown in Table 4 to estimate the population-level prevalence of different garage uses. We esti-
mated the frequency of each individual garage use as well as the frequency of each unique combination
of simultaneous garage uses (since respondents were allowed to select multiple concurrent uses). Where
respondents selected “Other” as one of their garage uses, we asked them to specify the other use. Because
all of the answers to the “other” category were elaborations or examples of the five closed-ended options,
we were able to re-code all of the “other” answers as one or more of the closed-ended answers provided.
For our off-street parking supply and surplus analysis, we used the weighted responses to the Table
4 questions to estimate the average off-street parking supply and surplus (available spaces minus house-
hold vehicles) for single-family detached homeowners in Sacramento. We also estimated the population-
level distribution of households across 10 levels of off-street parking sufficiency, ranging from a maxi-
mum surplus of six or more off-street parking spaces remaining to a maximum shortage of exceeding
off-street parking availability by three or more spaces. These estimates required three inputs—number
of vehicles parked at home per household, number of garage spaces available to each household, and
number of non-garage off-street parking spaces available to each household.
To calculate the number of vehicles and non-garage off-street parking spaces available to each re-
spondent household, we simply used their numeric responses to questions 3 and 4 in Table 4. To esti-
mate the number of “available” garage spaces, we used three scenarios based on respondents’ answers
to the first two questions in Table 4—nominal garage vehicle capacity and actual garage use. The first
scenario is a hypothetical “maximum supply” scenario, in which each household is assumed to have their
full garage capacity available for vehicle parking—i.e., 0, 1, 2, or 3 spaces based on respondents’ answers
to question 1—regardless of how respondents actually use their garage (question 2).
The second and third scenarios provide rough upper and lower estimates for actual garage parking
space availability, adjusting the nominal garage capacity based on how respondents report using their
garages. The “actual availability—high estimate” scenario assumes that households that did not indicate
using their garage for vehicle parking have zero garage spaces available. All other households are assumed
192 JOURNAL OF TRANSPORT AND LAND USE 15.1
to have their full garage capacity available for vehicular parking, even if the respondent reported using
the garage for other purposes in addition to car parking.
The “actual availability—low estimate” scenario takes the high estimate scenario and tightens its
assumptions. Like the high estimate scenario, the low estimate scenario assumes that households that
reported not using their garage for vehicle parking have zero garage spaces available. Households that
reported using their garage for vehicle parking only are similarly assumed to have their full garage capac-
ity available for parking. But households that reported using their garage for both vehicle parking and
another use were assigned just one available garage parking space (rather than the full nominal capacity,
as in the high estimate scenario). This scenario therefore represents a lower bound on the likely avail-
ability of garage parking.
Figure 1 illustrates how the different scenarios play out across a range of stated garage uses for a
hypothetical household that has a 2-car garage. The “Reality” column illustrates the unobserved real-
ity behind the corresponding answer in the “Stated Garage Use in Survey” column. The columns on
the right show how the stated use in the survey translates into our three scenarios. The hypothetical
maximum supply and high estimate scenarios risk overestimating the actual parking space availability in
reality, while the low estimate scenario risks underestimating it.
Reality
(Unobserved)
Stated Garage Use in
Survey
Hypothetical Maxi-
mum Supply
Actual Availability –
High Estimate
Actual Availability –
Low Estimate
Car parking only
Car parking and other
storage
Other storage only
Figure 1. Three scenarios of garage parking space availability
193Not enough parking, you say? A study of garage use and parking supply for single-family homes in Sacramento
3.3 Gauging on-street parking supply
To estimate the full effective parking supply available to Sacramento’s single-family homes, we also
needed to ascertain the on-street parking supply in those areas. We did that by adapting the method
from Guo et al. (2012) and Guo (2013), which is summarized above. Table 5 shows the numbers and
data sources we used to estimate the average number of on-street parking spaces for single-family de-
tached homes in Sacramento.
Table 5. Inputs for on-street parking supply estimate
Input Value Source
Lot size 6,806 sq. ft. (0.16 acres)Sacramento County (2020)
Frontage-depth ratio 1:3 Various
Parking space length 22 ft.Sacramento City Planning and Development Code
section 17.608.040(F)(2)(a)
Curb cut length 25 ft. (20 ft. * 1.25)Sacramento City Planning and Development Code
section 17.508.050; Thigpen & Volker (2017)
We calculated the 6,806 square-foot average parcel size for single-family detached homes in the city
from Sacramento County’s Open Data portal (Sacramento County, 2020). We used all (N=112,368)
parcels in the city that (1) were classified with a “most probable use” of Single Family—Subdivision, Sin-
gle Family—Non-subdivision, or Single Family—Planned Unit Development, and (2) were between
2,000 square feet—the lowest minimum lot size in any residential zone in the city (City of Sacramento,
n.d.)—and 1 acre (43,560 square feet) in size. We then estimated the average street line length for the
parcels using a frontage-depth ratio of 1:3. Rather than use Guo et al.’s (2012) assumed average ratio of
2:3 for newer single-family lots (with houses built between 1970 and 2010) across the US, we decided
to take a more context-sensitive approach. Our visual inspection of the parcel map maintained by the
Sacramento County Assessor, our discussions with local planners, and our own observations in the city
indicated that a 1:3 ratio was more representative of Sacramento lots. We also assume that only the front
of each lot has street frontage and on-street parking spaces. While many lots in Sacramento also back
onto alleys, “street” parking is usually not allowed in the alleys. Our method also discounts the street
line length for corner lots. Each of these assumptions deliberately result in more conservative estimates.
To estimate the average number of on-street parking spaces per single-family detached home,
we took the street line length, subtracted an assumed average curb cut length of 25 feet, and divided the
remainder by a parking space length of 22 feet. We derived the 25-foot curb cut length by conservatively
assuming each home has the widest driveway (20 feet; City of Sacramento Planning and Development
Code Section 17.508.050 (n.d.) (A)) allowed by code for a 2-car parking area (the most common garage
size in our weighted sample) and has another 5 feet (25%) of unusable curb space adjacent to the drive-
way. The 25% fudge factor is the same value used by Thigpen and Volker (2017) based on observations
of on-street parking conditions in residential neighborhoods in nearby Davis, California. The 22-foot
parking space length is based on the city’s minimum standards for (off-site) parallel parking spaces for
standard vehicles (City of Sacramento, n.d., Planning and Development Code Section 17.608.040 (F)
(2)(a)).
Overall, the assumptions undergirding our on-street parking calculus are intentionally conserva-
tive. We want to avoid overstating the effective parking supply (and any surplus) for single-family homes
and the related capacity for ADU development. However, it bears noting that single-family households
are not legally entitled to on-street parking on the public streets outside their homes. Their effective
on-street parking supply could decrease if the city began charging for parking or otherwise restricted
194 JOURNAL OF TRANSPORT AND LAND USE 15.1
their access to on-street parking, e.g., by creating a parking meter zone (California Vehicle Code Section
22508, n.d).
3.4 Putting it all together—Total supply, surplus, and ADU implications
We combined our off-street and on-street parking supply estimates to calculate the total effective park-
ing supply (available spaces) and surplus (available spaces minus household vehicles) per house for Sac-
ramento’s single-family homes across the three off-street parking supply scenarios discussed above—hy-
pothetical maximum supply, actual availability—high estimate, and actual availability—low estimate.
This, in turn, allowed us to explore the effect on parking supply of adding an ADU to the average
single-family detached home.
3.5 Limitations
Our findings are directly generalizable to the population of households that own single-family detached
homes in Sacramento. Furthermore, they provide evidence of likely patterns of garage use and effective
parking supply and demand in single-family neighborhoods in other urban areas with similar charac-
teristics (like weather, lot size, garage prevalence, on-street parking regulations, household size, vehicle
availability, the extent and quality of transit services, and other factors). There is good evidence that these
types of residential neighborhoods are pervasive: research using a variety of data sources and method-
ologies has arrived at a consensus that the type of lower-density, suburban development patterns that
characterize single-family neighborhoods are the most common land-use types in the US and globally
(Barrington-Leigh & Millard-Ball, 2015; Voulgaris et al., 2016; Wheeler, 2015). Furthermore, Census
data confirms that Sacramento is broadly similar to California and the US with respect to patterns of
household size and vehicle ownership for single-family detached homeowners. Of the 15 combinations
of household size and vehicle ownership we use in this paper for weighting purposes, the most the city
of Sacramento differs from California and the US is 3 and 5 percentage points, respectively, while the
majority of categories differ by one percentage point or less. If researchers or city planners want to learn
more about their particular city or region, they can also use this study's methodology in other settings.
Given the dearth of studies on these topics, future research in other geographies would further our un-
derstanding of how garage use and single-family parking sufficiency might vary by context.
Future research is also needed to empirically quantify how parking demand from both ADUs and
non-residents affects on-street parking occupancy. Our study assesses hypothetical effects only, using es-
timates of on-street parking supply from citywide data and estimates of occupancy rates and ADU-gen-
erated parking demand from previous studies. We could not calculate actual on-street parking supply or
occupancy for the homes in our study—let alone discern where the parked cars come from—because
our survey data does not include street-level geographic identifiers.
We use post-stratification weighting to bring a non-probability sample in line with the population
of Sacramento single-family homeowners. This approach relies on the assumption that participants and
non-participants within each “cell” (particular combination of household size and cars available) have
the same distribution of the outcome variables (parking availability and use). If this assumption holds,
it reduces selection biases. Many other studies of residential car parking, especially of garage use, rely on
small or non-probabilistic samples, and few attempt to correct for misalignment of the sample with the
population of interest.
195Not enough parking, you say? A study of garage use and parking supply for single-family homes in Sacramento
4 Results
4.1 Garage use
We first present our findings on garage capacity and use, since they are key to our estimates of effective
parking supply and sufficiency. Our weighted estimates show that the vast majority of single-family de-
tached households in Sacramento have garages (94.1%), with an average nominal (built) capacity of 1.7
cars. Table 6 and Figure 2 show the overall frequency of individual garage uses among those households.
The most common individual use is for storage, with over 75% of households using their garages for this
purpose. This exceeds the 63% of households that use their garages at least partially for their intended
purpose, car parking. Another somewhat common use was as an extra room (12%), while the other use
cases (as a living space used by the household or rented to a tenant) were much less common.
Table 6. Percent of garages used for different individual purposes
Garage use Estimate S.E.
Storage 75.5%2.60
Car Parking and Repair 63.0%2.92
Extra Room 12.1%1.99
Household Living Space 3.1%1.20
Rented Living Space 1.7%1.00
Figure 2. Percent of garages used for different individual purposes
Note: Error bars depict the 95% confidence intervals
Table 7 and Figure 3 show the prevalence of different combinations of simultaneous garage uses
amongst single-family households with garages. We find that the most common use combination is for
both car parking and storage (40%). Interestingly, more respondents indicated using their garage for
storage only (27%) than for car parking only (18%). The next most common uses were as an extra room
(not as a living space) only (5%) and as an extra room and storage (4%). The remaining combinations all
had low frequencies. We examined the association of garage use with various household characteristics
and found no strong associations.
196 JOURNAL OF TRANSPORT AND LAND USE 15.1
Table 7. Percent of garages used for different combined purposes
Garage use combinations Estimate S.E.
Car Parking and Storage 39.9%3.04
Storage only 26.6%2.71
Car Parking only 18.4%2.16
Extra Room only 4.7%1.54
Storage and Extra Room 3.5%0.89
Combinations with only 1 response 3.0%1.17
Car Parking, Storage, and Extra Room 2.7%0.97
Car Parking, Storage, and Household
Living Space
0.7%0.43
Car Parking and Extra Room 0.5%0.31
Figure 3. Percent of garages used for different combined purposes
Note: Error bars depict 95% confidence intervals
4.2 Off-street parking supply and sufficiency
Looking at the total supply and sufficiency of off-street parking supply—garage spaces plus other off-
street spaces—we find that most single-family detached households in Sacramento have sufficient off-
street capacity to park all their vehicles. On average, households had 2.3 vehicles and between 2.9 and
3.8 total off-street spaces, depending on the scenario. Overall, parking supply sufficiency ranged from a
surplus of 6+ off-street spaces to a shortage of 3 spaces. Table 8 and Figure 4 show our population-level
estimates for the percent of households across the 10-level range of parking sufficiency for each of our
three scenarios. These estimates are based on our full sample, including households without garages.
In the hypothetical “maximum supply” scenario with all garage spaces available for car parking,
only 8% of households would need to park a car(s) on the street. The remaining 92% of households
would have sufficient off-street parking supply to store their car(s). About one in five (20%) households
have as many cars as theoretically-available off-street spaces, and the majority (64%) have 1 to 3 extra
off-street spaces remaining.
In the more realistic scenario with an upper-end estimate for available garage spaces (actual avail-
ability—high estimate), the percent of households that have more cars than off-street spaces nearly
doubles from 8% in the hypothetical scenario to 15%. The percentage of households with as many cars
as off-street spaces also increases from 20% to over a quarter of households (29%). But a majority (50%)
still have 1 to 3 extra off-street spaces remaining.
In the more realistic scenario with a lower-end estimate for available garage spaces (actual availabil-
197Not enough parking, you say? A study of garage use and parking supply for single-family homes in Sacramento
ity—low estimate), approximately one in four households (25%) have more cars than off-street parking
spaces, which is similar to Taylor’s (2020) estimate from Melbourne (26%). The percent of households
with equal numbers of cars as off-street spaces remains at 29%, and the percent of households with 1 to
3 remaining off-street spaces declines to 42%.
Table 8. Percent of households with off-street parking spaces remaining
Hypothetical
Maximum Supply
Actual Availability –
High Estimate
Actual Availability –
Low Estimate
Off-street spaces remaining Estimate S.E.Estimate S.E.Estimate S.E.
6 or more off-street spaces remaining 1.0%0.5 1.0%0.5 1.0%0.5
5 off-street spaces remaining 1.9%0.9 1.2%0.5 0.7%0.4
4 off-street spaces remaining 5.1%1.2 3.5%0.8 2.3%0.8
3 off-street spaces remaining 14.9%2.0 10.7%1.9 3.9%1.1
2 off-street spaces remaining 27.4%2.5 18.4%2.1 15.5%2.0
1 off-street spaces remaining 21.6%2.5 20.8%2.4 22.7%2.3
Off-street capacity full 20.0%2.6 29.3%2.9 29.1%2.9
Exceeds off-street capacity by 1 5.0%1.6 9.5%2.1 18.0%2.6
Exceeds off-street capacity by 2 0.8%0.3 3.4%1.2 4.5%1.5
Exceeds off-street capacity by 3 2.1%1.2 2.2%1.2 2.2%1.2
Figure 4. Percent of households with off-street parking spaces remaining
198 JOURNAL OF TRANSPORT AND LAND USE 15.1
4.3 On-street parking supply
Using the inputs listed in Table 5, we estimated that the average single-family detached home in Sacra-
mento has approximately 48 feet of street frontage. We then subtracted 25 feet to account for a drive-
way curb cut (20 feet) and potentially unusable curb space (5 feet), yielding 23 feet of curb space. That
translates to almost exactly one on-street parking space per average single-family detached home, using
a parking space length of 22 feet.
4.4 Total effective parking supply and sufficiency
Table 9 shows the average total effective parking supply (available off-street and on-street spaces com-
bined) and surplus (available spaces minus available household vehicles) per single-family home across
the three off-street parking supply scenarios. The average single-family detached household in Sacra-
mento has access to between 1.6 and 2.0 more parking spaces than it has vehicles, according to our
low and high estimates of actual (realistic) parking space availability, respectively. The average surplus
increases to 2.5 spaces under the hypothetical maximum.
Achieving the hypothetical maximum might not be feasible, since it would require every household
to clear enough space in their garage to accommodate its nominal vehicular capacity—i.e., each 1-car
garage could actually fit 1 car, each 2-car garage could actually fit 2 cars, etc. That would mean displac-
ing storage, home gyms, home offices, and even dwelling areas in some cases, which many homeowners
likely have no self-interest in doing. In any case, our results indicate that Sacramento’s single-family
detached homes have a substantial surplus of parking spaces under any of the three scenarios.
Table 9. Total effective parking supply and surplus per home
Scenario Average on-
street spaces
Average off-
street spaces
Average total
spaces
Average ve-
hicles available
Average surplus
spaces
A B A + B = C D C – D = E
Hypothetical maximum
supply
1.1 3.8 (0.09)4.9 (0.09)2.3 (0.05)2.5 (0.10)
Actual availability – high
estimate
1.1 3.3 (0.10)4.3 (0.10)2.3 (0.05)2.0 (0.10)
Actual availability – low
estimate
1.1 2.9 (0.09)3.9 (0.09)2.3 (0.05)1.6 (0.09)
Note: Sums may be off by one tenth due to rounding.
5 Discussion
5.1 Garage use
Our study expands the small, yet slowly growing, body of research on off-street parking use by reveal-
ing the multifaceted ways that Sacramento homeowners use their garage space: primarily for storage,
secondarily for car parking, and marginally for other purposes, like living spaces. Our research improves
on earlier work by explicitly allowing respondents to select multiple simultaneous garage use categories,
reflecting the multi-dimensional use cases seen in many homes (Taylor (2020) being a notable exception
to this pattern). However, this makes direct comparisons with previous findings more difficult, as does
the fact that the study setting is novel. Nonetheless, it is useful to assess the consistency of findings as we
seek to build the evidence base on garage uses.
199Not enough parking, you say? A study of garage use and parking supply for single-family homes in Sacramento
Our results align well with the general pattern of previous findings that the two main garage uses
are storing personal belongings and parking cars. An overwhelming majority of Sacramento single-
family homeowners (76%) use their garage for storage, in rough agreement with the qualitative results
of the Guo et al. (2012) expert opinion survey. By contrast, our estimate of the percent of Sacramento
single-family households using their garage for car parking (63%) was generally higher than studies in
cities such as San Francisco (51%) (Brown, 2007), Melbourne (41%) (Taylor, 2020), UK cities (36-
45%) (United Kingdom Department for Transport, 2007), and especially New York City (13%) (Guo,
2013). Only one previous study had a similarly high estimate (62%) to ours, and that was in a suburb
of Reading, England (Jenks & Noble, 1996, as discussed in Guo & Schloeter, 2013, 460). This pattern
of results from the literature suggests that residents of cities with older and smaller housing stock may be
enticed to convert their garages to other purposes and park their car elsewhere.
Our estimate of the percentage of households using their garage as a living space, either for the
household or rented to others (3.7%), roughly matches the findings in Chavez and Quinn’s systematic
field observations (3.2%; Chavez & Quinn, 1987), while falling much lower than the more informal
estimates of Wegmann (2015) and of the City of Compton city planner (Garrison, 2009). Though the
disparities could be attributable to differences in the study setting, it is also notable that the two studies
using formal quantitative methods and statistical sampling techniques yielded similar results.
5.2 Surplus parking spaces available to single-family homes
Our study provides the first estimate we have seen of the total effective parking supply and use for single-
family detached homes. Our findings indicate that Sacramento’s single-family homes have a sizeable
surplus of parking. We estimate that the average single-family detached homeowner in Sacramento
has access to between 1.6 and 2.0 more parking spaces than their household has vehicles, using our
low and high estimates of actual parking availability, respectively (Table 9). Our findings are consistent
with other indications that parking tends to be oversupplied in single-family neighborhoods, including
studies showing low occupancy rates for on-street parking (Roth, 2016; Schlossberg & Amos, 2015;
Thigpen & Volker, 2017) and the sufficiency of off-street parking for most households (Taylor, 2020).
The surplus could be even greater in areas with larger average lot sizes. For example, Guo et al.
(2012) estimated that new single-family homes built in the US between 1970 and 2010 had a hypo-
thetical maximum of 8-10 total parking spaces on an average lot size of 0.34 acres. Both numbers are
more than double their respective values in Sacramento, where we estimated a hypothetical maximum
of 4.9 parking spaces and an average lot size of 0.16 acres. However, Guo et al. (2012) did not estimate
parking surplus.
Another way to conceptualize the surplus is as the ratio of total parking spaces to vehicles available
to single-family detached homeowners. This compares to the more widely reported ratio of all parking
spaces (residential, commercial, and otherwise) to all passenger vehicles. The oft-cited “rule of thumb”
estimate for the US is 8 parking spaces per vehicle, but survey-based estimates have been considerably
lower. (Chester et al., 2010), for example, calculate a ratio of 3.4 to 1, combining separate survey-based
estimates for urban areas and rural areas. For single-family detached homes in Sacramento, we estimate
that there are between 1.5 and 1.9 residential parking spaces per vehicle on average, using our low and
high estimates of actual parking availability respectively.
5.3 Implications for ADUs
Under any of our three scenarios, the parking surplus for the average single-family detached home
in Sacramento is more than enough to accommodate the average ADU tenant. Households living in
200 JOURNAL OF TRANSPORT AND LAND USE 15.1
ADUs have an average of just one vehicle available to them, according to studies in Portland (Brown
& Palmeri, 2014; Gebhardt et al., 2018) and the San Francisco Bay Area (Chapple et al., 2012; City
of Sausalito, 2011; Wegmann & Chapple, 2012). Allocating one parking space for the ADU tenants
would still leave the average single-family home with 0.6 to 1.0 surplus parking spaces at periods of
maximum residential utilization (when all the vehicles available to the primary home and the ADU
were parked at home at the same time), using our low and high estimates of actual parking availability.
Scaling our findings up to the block level, a hypothetical 500-foot block face with 10 single-family
houses and no ADUs would have about 16 to 20 (rounded) surplus parking spaces not used by the
households that live there. Figure 5, panel A shows a conservative version of this scenario, where we al-
locate to each house 3 off-street parking spaces (in between our low and high estimates in Table 9, but
closer to the low estimate), 1 on-street space (rounded down from our estimate in Table 9) and 2 or 3
vehicles (totaling 23 vehicles across the block to match the weighted average of 2.3 vehicles per house-
hold from our sample). For simplicity, we assume that all household vehicles are parked on the block
during a period of maximum residential utilization (like the middle of the night), that households will
park their vehicles off the street as long as spaces remain available, and that there are no non-resident
vehicles parked on the block. Occupied parking spaces are shown as red-filled rectangles and vacant
spaces are shown as dashed white rectangles. For ease of visualization, all off-street parking spaces are
shown in front of the garage. In reality, our results show that about 1 vehicle would be parked in the
garage for the average single-family detached home in Sacramento. In total, there are 23 occupied off-
street spaces, 7 vacant off-street spaces, and 10 vacant on-street spaces under this scenario, for a total of
17 surplus spaces.
Figure 5. Block-level parking occupancy scenarios
How much does the surplus change if we add in ADUs? A recent study estimated that nearly 50%
of single-family detached homeowners in Sacramento could be open to building an ADU (Volker &
Handy, in press). Using 50% as a maximum ADU adoption scenario, we will assume that five of the 10
households on our hypothetical block have ADUs, with each ADU generating one vehicle (in line with
201Not enough parking, you say? A study of garage use and parking supply for single-family homes in Sacramento
the average from the literature). Figure 5, panel B illustrates this scenario. We use the same assumptions
as the scenario in panel A with one difference—we allocate one additional vehicle to every other house
on the block (for a total of 28 resident vehicles on the block). The houses with the additional vehicle are
indicated by the ADU (gray rectangle) in the backyard. In total, there are 26 occupied off-street spaces,
1 occupied on-street space, 3 vacant off-street spaces, and 9 vacant on-street spaces under this scenario,
for a total of 12 surplus spaces.
But what if the ADUs also reduced neighborhood parking supply? Let us assume for the sake of
argument that every ADU in the previous scenario was converted from a garage. Our weighted estimates
show that the average single-family homeowner in Sacramento stores between 0.7 and 1.1 vehicles in
their garage, based on our low- and high-availability scenarios respectively. Using the higher estimate,
each ADU would remove 1.1 parking spaces in addition to adding one vehicle to the total neighbor-
hood parking demand. Figure 5, panel C illustrates this scenario, though for simplicity it uses a round
number of 1 vehicle per garage rather than our 1.1-vehicle high estimate. We use the same assumptions
as the scenario in panel B except we remove one off-street parking space from each house with an ADU
(depicted as a gray ADU rectangle where the garage and accompanying off-street parking space used
to be). In total, there are 22 occupied off-street spaces, 6 occupied on-street spaces, 3 vacant off-street
spaces, and 4 vacant on-street spaces, for a total of 7 surplus spaces.
Under all three hypothetical scenarios shown in Figure 5 there would be numerous vacant on-street
spaces on the 10-house block during periods of maximum residential occupancy. Of course, this does
not mean that there would always be empty on-street parking spaces on the block. Even where the total
number of parking spaces (off-street and on-street) exceeds the number of vehicles available to residents
on the block, the cumulative parking demand from the residents plus visitors, home help, and other
non-residents could exceed the total supply. This could be especially problematic on blocks with retail or
commercial uses or within walking distance of concentrated retail, commercial, or recreational activities.
However, previous censuses of on-street parking in single-family neighborhoods indicate that on-
street parking is vastly oversupplied even during times of peak occupancy, suggesting our conservative
assumptions might result in an overestimate of on-street parking impacts. Thigpen and Volker (2017),
Schlossberg and Amos (2015), and Roth (2016) all found vacancy rates between 71% and 89%. Fur-
thermore, local governments have numerous tools available to help ensure that residents of high-use
areas have enough parking without requiring that homeowners add off-street parking for their ADUs
or replace off-street parking that was converted into an ADU (Brown et al., 2017). They can create a
residential parking permit district to restrict longer-term on-street parking to permitted residents, like
Sacramento and most other large cities in the United States have already done in many places (City of
Sacramento, 2021c). They can also allow residents to park in front of their own driveways. The Califor-
nia Vehicle Code, for example, allows local governments to “authorize the owner or lessee of property
to park a vehicle in front of the owner’s or lessee’s private driveway when the vehicle displays a permit
issued pursuant to the ordinance authorizing such parking” (California Vehicle Code Section 22507.2,
n.d.). In the case of Sacramento, the additional space provided by blocking the driveway would double
the number of on-street spaces available to the average single-family home (according to our estimate of
1.05 spaces), and would effectively replace any on-street parking used by non-residents.
Overall, our findings tend to belie claims that ADUs will overwhelm existing parking supplies in
single-family neighborhoods. They also cast doubt on the necessity of off-street parking requirements
for ADUs.
202 JOURNAL OF TRANSPORT AND LAND USE 15.1
6 Conclusion
In this article, we use a survey of homeowners in Sacramento, California to investigate the supply and
sufficiency of residential parking supply in single-family neighborhoods, including how households
actually use their garages. We use our findings on these sorely understudied topics to examine one of
the most commonly cited reasons for opposing construction of accessory dwelling units in single-family
neighborhoods—that the new residents would overwhelm scarce on-street parking. We find that 63%
of single-family detached homeowners in Sacramento use their garage at least partially for car parking.
However, only 18% use their garage solely to park cars. On the other hand, 27% of households use
their garage exclusively for storage. With respect to parking sufficiency, we find that, even under our
most conservative scenario, 75% of households have enough off-street parking to park all their vehicles.
Combining off-street and on-street parking supplies, we find—again under our most conservative sce-
nario—that Sacramento’s single-family detached households have, on average, 1.6 more parking spaces
available to them than they have vehicles. That surplus is more than enough to accommodate the aver-
age ADU tenant and their vehicle. That belies claims that ADUs will overwhelm existing parking sup-
plies in single-family neighborhoods.
Acknowledgments
The authors thank the National Center for Sustainable Transportation for providing crucial funding
for this research through a Dissertation Fellowship to Dr. Volker. Thank you also to the editors and the
reviewers for their constructive feedback, which improved the paper's focus and quality.
203Not enough parking, you say? A study of garage use and parking supply for single-family homes in Sacramento
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