Gattis, Dammalapati, Cotton, Cotton
3rd Urban Street Symposium
1
June 24-27, 2007 Seattle, Washington
CROSS SECTION WIDTH FOR PARALLEL PARKING
by
J. L. Gattis, Ph.D., P.E.
Srinivas Dammalapati
Joshua Cotton
Joseph Cotton
Corresponding Author:
J. L. Gattis, Ph.D., P.E.
Mack-Blackwell Transportation Center
4190 Bell Engineering Center
Fayetteville, AR 72701
voice (501)575-3617
fax (501)575-7168
jgattis@uark.edu
ABSTRACT
The growth of alternative roadway design approaches, such as Traditional Neighborhood Design
and Context Sensitive Design, has focused new attention on urban street design standards. Some
have called for urban street cross sections that are narrower than those recommended in
established design references. One cross section design element for which there is some
variation among different published guidelines and practices is the width provided for on-street
parallel parking. This paper reports the findings from field measurements made to determine
how much width was actually occupied by parallel-parked passenger cars, both in commercial
and in residential areas.
Gattis, Dammalapati, Cotton, Cotton
2
CROSS SECTION WIDTH FOR PARALLEL PARKING
by J. L. Gattis, Ph.D., P.E., Srinivas Dammalapati, Joshua Cotton, Joseph Cotton
INTRODUCTION
The growth of alternative roadway design approaches, such as Traditional Neighborhood Design
and Context Sensitive Design, has focused new attention on urban street design standards. Some
adherents of these newer, alternative design perspectives have proposed and adopted urban street
cross sections that are not as wide as some of the cross sections based on recommendations
found in established references. One of the cross sectional elements about which the various
references disagree, and for which one can find a range of recommended minimum acceptable
values, is the width allowed for and allocated to on-street parallel parking.
The lack of agreement about the width needed to accommodate on-street parallel parking
prompted a study to determine how much width was actually being occupied by parallel-parked
vehicles, both in commercial and in residential areas. Those involved in the study made field
measurements of parallel-parked vehicles.
Except where otherwise stated, the scope of the literature review and the field studies was
confined to the passenger car or P design vehicle, which reflects the limiting attributes of most
standard passenger cars (PC) such as sedans, coupes, station wagons, as well as minivans, sportutility vehicles (SUV), and pickup trucks (PU). It represents most of the non-commercial vehicle
fleet that the general public owns and drives.
LITERTURE REVIEW
A review of available literature helps illustrate divergent points of view related to cross sectional
width needed for on-street parallel parking. Both traditional and alternative sources were
reviewed.
Traditional Design Criteria
The chief sources of “traditional” street cross section design criteria are the policy publications
by AASHTO (American Association of State Highway and Transportation Officials), and the
Manual on Uniform Traffic Control Devices (MUTCD) by the Federal Highway Administration.
A Policy on Highway Types (Geometric), published by American Association of State
Highway Officials (AASHO) in 1940 (1) stated the following about parking.
“The width of parking lane depends upon the proposed method of parking and
character of traffic. For parallel parking of passenger vehicles a width of 7 feet
has been used extensively. It may be wide enough for side streets where few
trucks use the streets and travel is at low speed. On through streets, lanes for
parallel parking of passenger vehicles should be at least 8 feet wide if the adjacent
traffic lanes are to be utilized without encroachment onto other traffic lanes.”
The focus of the 1957 AASHO Red Book (2) was urban freeways and arterials. Unlike
the later AASHTO Green Books, it did not contain chapters on local or collector street design.
The lowest category listed in the table of contents was “major streets”. The discussion of onstreet parking for passenger cars on major streets included the following.
“Passenger vehicles now in use are 5 ft.-8 in. to 6 ft.-10 in. in width. A vehicle
parked alongside a curb likely will not be stopped against the curb but with the
right edge 4 to 8 inches from it. Or it may be at a slight angle so that one
Gattis, Dammalapati, Cotton, Cotton
3
extremity is 6 to 12 inches from the curb. Thus, the actual street space occupied
6.5 to 7.5 feet from the curb.”
The discussion proceeded to suggest an additional clearance of 3 to 5 ft (0.9 to 1.5 m), for a total
parking lane width of 10 to 12 feet (3.0 to 3.6 m). The 1973 Red Book (3), while briefly
mentioning some aspects of collector and local streets, also did not contain chapters addressing
design of these lesser functional classes.
In the “Cross Section” chapter, the 1990 (4) and the 1994 (5) Green Books suggested an
8 ft (2.4 m) “desirable minimum width of a parking lane”. This dimension was recommended
for both arterial and collector streets. The discussion noted that a width of 10 to 12 ft (3.0 to 3.6
m) was desirable to provide greater clearance, and this wider dimension should be used when
bike routes are adjacent to the parked vehicles, to permit bicyclists to pass by open car doors. In
the “Local Roads and Streets” chapter, AASHTO (5) mentioned a cross section width to
accommodate a 12 ft (3.6 m) wide center travel lane with 7.2 ft (2.2 m) wide parking lane on
either side. Lanes on the local street were unmarked. The “Collector Roads and Streets” chapter
listed “a parallel parking lane from 7 to 10 ft (2.1 to 3.0 m) in width” in residential areas, and 8
to 10 ft (2.4 to 3.0 m) in commercial areas.
The 2001 or fourth edition (6) of the Green Book stated that for urban arterial roadways,
a “parking lane width as narrow as 2.4 m (8 ft) may be acceptable” and recommended a width of
3.0 to 3.6 m (10 to 12 ft). On urban collector streets in residential areas, a width of 2.1 to 2.4 m
(7 to 8 ft) was suggested to accommodate on-street parking. The chapter on local streets gave a
width for residential area parking of 2.2 m (7 ft) in one place and a minimum of 2.1 m (7 ft) in
another. An 8 ft (2.4 m) width was the suggested minimum width in commercial areas. The
2004 Green Book “Cross Section Elements” chapter and other passages (7) contain similar
statements. The discussion noted that a width of 10 to 12 ft (3.0 to 3.6 m) was desirable to
provide greater clearance, and this wider dimension should be used when bike routes are
adjacent to the parked vehicles, to permit bicyclists to pass by open car doors.
A 7 ft (2.1 m) parking width is mentioned in the 1948 Manual on Uniform Traffic
Control Devices (MUTCD) (8). The 1961 (Figure 2-10) (9), 1971 (Figure 3-16) (10), 1978
(Figure 3-16) (11), 1988 (Figure 3-16) (12), 2001 (Figure 3B-17) (13) , and 2003 (Figure 3B-18)
(14) editions of the MUTCD showed an 8 ft (2.4 m) wide marked parallel parking space.
Other Design Guides
By their very nature, AASHTO and FHWA publications are positioned to have a wider
distribution and acceptance than many other design guides are. Some alternative design guide
sources were identified and reviewed.
Residential Streets, jointly published in 1990 (15) by American Society of Civil
Engineers (ASCE), National Association of Home Builders (NAHB), and the Urban Land
Institute (ULI), showed a parking lane width of 6 to 7 ft (1.8 to 2.1 m) on an “access” street, and
an 8 ft (2.4 m) width for subcollector and collector streets. The document states that for
residential streets, “Designers should select the minimum width that will realistically satisfy all
realistic needs.” A moving lane of 10 ft (3.0 m) width was shown for access, subcollector, and
collector streets, for a total street width of 22 to 24 ft (6.6 to 7.2 m) for access streets, 26 ft (7.8
m) for subcollector streets, and 36 ft (10.8 m) for collector streets.
The third edition of Residential Streets, published in 2001 (16), showed a parking lane
width dimension of 6 to 7 ft (1.8 to 2.1 m) for the local streets and 8 ft (2.4 m) for residential
collector streets. A moving lane of 11 to 12 ft (3.3 to 3.6 m) width was shown for local streets
Gattis, Dammalapati, Cotton, Cotton
4
with parking not expected or restricted to one side and a 10 to 14 ft (3.0 to 4.2 m) wide moving
lane for local streets with parking on both sides. The widths recommended in the third edition
were somewhat greater than those in the second edition.
A guide prepared for the state of Florida (17) promoted a number of relatively narrow
street width design standards. It listed “7- or 8-foot parking lanes” for collectors.
Design drawings prepared for the Portland metropolitan region (18) showed a width of 7
ft (2.1 m) allocated for parking in a number of streetscapes, including commercial areas. The
design guidelines state “The preferred on-street parking lane width for parallel parking is 7 feet.”
The Main Street handbook developed by the Oregon Department of Transportation (19)
for Oregon communities mentioned a parallel parking width of 7 ft (2.1 m) on main streets. It
also reported a scenario in which to increase the sidewalk width the parking width was reduced
from 8 ft (2.4 m) to 7 ft (2.1 m).
One of a series of articles discussing various aspects of traffic calming in an ITE Journal
listed a minimum 7.2 ft (2.2 m) parking lane width for local streets, and an 8 ft (2.4 m) minimum
for collectors (20). In another article (21), authors reported that a street standard for their city
had been modified to reduce the parking lane width from 2.4 m (8 ft) to 2.1 m (7 ft). Given the
context of the article, it can be assumed the reduced width applied to a residential street cross
section.
RESEARCH OBJECTIVES AND METHODS
The objectives of the study were simple and straightforward.
1.
measure the actual cross section width occupied by parked vehicles, in both commercial
and residential environments
2.
compare the measured dimensions with those proposed in various sources
The data were collected in two cities, Mobile, Alabama and Fayetteville, Arkansas. Mobile is an
older, established city with a population of about 200,000, in a growing metropolitan area of
about 500,000 people. Fayetteville is a city of over 60,000 near the south end of a metropolitan
area of over 200,000. In the 1990s, the area was one of the fastest growing metropolitan areas in
the country.
Street Environment and Data Pool
The data were collected in downtown or older near-downtown commercial areas, in older
residential neighborhoods, and near a university campus. These areas were targeted because onstreet parking regularly occurs on the streets in these areas. Many of these streets also possessed
another attribute desired for the study, being rather narrow. It was hypothesized on a narrow
street, any given driver would be less likely to position their parked vehicle randomly, and more
likely to attempt to park close to the curb.
The number of parked vehicles in the sample simply reflects the number of vehicles that
were parallel-parked along the streets at the time the measurements were made. As data were
collected, the vehicles were classified as either passenger car (PC), sport utility vehicle or van
(SUV/Van), or pickup (PU). Table 1 lists the streets on which parked vehicle offsets from the
curb were measured, and what types of vehicles were encountered.
In the commercial areas, the individual parking spaces were delineated or marked. The
streets also had marked center and/or lane lines. In the residential areas, the on-street parking
spaces were not marked, and most of the studied residential area streets had no center or lane line
Gattis, Dammalapati, Cotton, Cotton
5
markings. As the table shows, some parking spaces in the campus area were marked but most
were not; center and lane lines were not present.
TABLE 1 Description of Streets and Vehicle Types
Street Name
Nominal
Width
(ft)
Parking
Lanes
No.of
Marked
Moving
Lanes
Vehicle types
PC
SUV/ PU
Van
Commercial streets - Mobile
E. Daulphin St.(north) 16.3
E. Daulphin St.(south) 20.7
N. Jackson St.
21.5
Royal St. (east)
26.7
Royal St. (west)
26.7
State St.
22.9
1
1
1
1
1
1
2
2
2
2
2
2
6
32
2
1
4
6
2
16
1
1
3
1
0
6
1
1
2
3
Commercial streets - Fayetteville
N. Block
22.0
E. Center
19.7
W. Center
20.0
N. East (south)
25.7
N. East (north)
16.3
E. Mountain
23.0
W. Mountain
22.3
Dickson Street
24.0
2
2
2
2
2
2
2
2
2
2
2
2
1
2
2
1
8
23
7
5
20
20
10
27
6
6
0
2
5
4
2
12
5
8
1
1
5
6
1
6
Residential streets - One-way
Meadow
25.7
Watson
22.3
1
1
none
none
11
5
2
1
3
0
Residential streets - Two-way
Boles
19.7
Lafayette
40.7
Sutton
23.7
Washington
30.7
Walnut
19.7
Willow (north)
27.0
Willow (south)
20.7
1
2
1
2
1
1
1
none
2
none
none
none
none
none
9
11
7
9
8
6
6
1
2
1
4
3
1
3
2
4
3
2
2
0
1
Campus – marked spaces
Ark. Avenue (east)
Ark. Avenue (west)
26.0
26.0
1
1
1
1
19
23
11
10
2
6
Campus - unmarked spaces
Douglas
Lafayette
Lindell
Oakland
Storer
24.0
28.3
27.8
27.8
20.1
1
1
2
2
1
none
2
none
none
none
14
9
22
26
6
8
4
7
9
4
8
3
10
11
3
NOTE: “Nominal width” where parking lanes were marked was measured between or
inside the parking lanes; where there were no parking space markings, width
was measured between curb faces
Gattis, Dammalapati, Cotton, Cotton
6
The streets on which data were collected in Fayetteville are in level to slightly rolling
terrain. Curb faces are vertical, and curb heights typically do not exceed 6 in (150 mm).
Extreme vertical differences between elevations of the street surface and the gutter surface,
which could cause drivers to shy away from the curb, were typically not observed. Speeds on
these streets are typically in the 20 to 35 mph (30 to 55 km/h) range, and traffic volumes were
well below those that would cause congestion.
Data Collection
Two-person teams measured the offset distance from the curb face to the bottom of the side of
the tire toward or facing the moving traffic. Measurements were made to the tire in order to
avoid coming in contact with any part of the vehicle other than the bottom of the tire. A pole
was employed to hold the tape end next to the tire. This allowed the team-member working on
the moving-traffic side of the parked vehicle to stand up while making the measurement, and
avoid bending over. Measurements were made at both front and rear tires, with the greater of the
two offset distances used in subsequent data analysis. All measurements were made during the
daytime in clear, dry weather. On one-way streets, dimensions for vehicles parked on both sides
of the street were taken.
Measuring to the tire does not reflect the entire width occupied by the parked vehicle,
since the edge of the vehicle body may extend past the tire. To estimate a total offset (i.e., the
entire width occupied by a parked vehicle), a four-person crew separately measured 50 vehicles
parked on level terrain. A tape was stretched from the outer edge of the left front tire to the outer
edge of the left rear tire. Then, a surveying pole with a bulls-eye level attached was placed in a
vertical position extending from the pavement surface up to the vehicle body, and the offset
distance from the pole to the stretched tape was measured. Distances from the tire edge to the
both the edge of the vehicle body and to the outer edge of the left-side mirror were read.
Measurements were made to the nearest 0.5 inch (13 mm). These dimensions were evaluated
and standard values for the additional width out to the edge of the body and out to the edge of the
mirror were determined. After other analyses had been performed, these standard values were
added to measurements made from the curb to tire.
Data Analysis
Offset distances from the curb face to the outer edges of the left tires were measured for 88
vehicles parked in marked spaces in a commercial area of Mobile, Alabama. In Fayetteville,
Arkansas, offsets of 202 vehicles in marked spaces in two commercial areas, 107 vehicles in
unmarked spaces in residential areas, 71vehicles in marked spaces in a campus area, and 144
vehicles in unmarked spaces in a campus area were measured. The three handicapped spaces
and the nine vehicles which were parked on a one-way street with no marked stalls on the other
side of the street were excluded from the commercial-area data set of Fayetteville, Arkansas,
which left 190 vehicles in that commercial area.
Statistical tests were performed to determine if certain differences were statistically
significant, with ∀ = 0.05. The following t-test equation comparing means from two samples of
unequal size, where N is sample size and s is sample standard deviation, was used (22).
2
offset 1 − offset
2
2
s1 s 2
= t ×
+
N1 N 2
Gattis, Dammalapati, Cotton, Cotton
7
Table 2 describes the tested pairs and the outcome of the test for each pair. Based on these
outcomes, the data from the campus area were kept separate from the residential area, while the
data from the two Fayetteville commercial areas were combined.
TABLE 2 Tests for Significant Differences Between Area Types
Comparing the two means of
Mean
1
Mean
2
Was the difference
significant?
Mobile commercial vs.
Fayetteville commercial downtown
6.81
6.93
no
Fayetteville commercial:
downtown vs. near-downtown
6.93
6.84
no
Mobile commercial vs
Fayetteville commercial near-downtown
6.81
6.84
no
Campus area unmarked vs.
Residential areas unmarked
6.07
6.21
yes
Table 3 shows the distribution of the three vehicle groups. Note the higher percentages
of SUV/van in the campus-marked area, and the higher percentage of pickup trucks in the
campus-unmarked area.
As a check, data were examined to ascertain if any of the three vehicle classes were overor underrepresented on any of the street widths. Commercial and residential streets were
analyzed separately. The Chi-square test of independence was performed on a contingency
table. The numbers of vehicles within each class were aggregated by 1.0 ft (0.3 m) width
increments. The outcomes of both the commercial and the residential group Chi-square tests
found independence. That is, the numbers of each of the three vehicle classes were sufficiently
proportional in each of the width increments.
That fraction of the total street cross section intended for parking may be marked, or it
may be unmarked and as such be co-mingled with overall street or outer lane width. In order to
examine parking offset as a function of traveled-way width, some simplifying assumptions about
streets with no marked center or lane lines were made. Based on knowledge of traffic on each
street, the number of lanes that can and do normally move simultaneously on those streets
lacking center or lane line markings was identified. A width of 7.5 ft (2.3 m) for each lane of onstreet parking that did occur was subtracted from the total street width. The remaining width was
divided by number of lanes that can be observed moving when parking is present to arrive at the
width of a moving lane. The assumed 7.5 ft (2.3 m) width was chosen as a “mid-way” value
between the traditional 8.0 ft (2.4 m) width and the commonly-proposed alternative 7.0 ft (2.1 m)
parking lane width.
Gattis, Dammalapati, Cotton, Cotton
8
TABLE 3 Descriptors by Area and Vehicle Group
Area type
Vehicle group
N
%
Mean
offset
(ft)
Standard.
deviation
(ft)
Commercial
marked
Passenger Car
SUV/Van
Pickup (PU)
All Combined
171
61
46
278
62%
22%
17%
100%
6.79
6.97
7.06
6.88
0.60
0.65
0.50
0.61
Residential
unmarked
Passenger Car
SUV/Van
Pickup (PU)
All Combined
72
18
17
107
67%
17%
16%
100%
6.15
6.25
6.43
6.21
0.55
0.30
0.56
0.53
Campus
marked
Passenger Car
SUV/Van
Pickup (PU)
All Combined
42
21
8
71
59%
30%
11%
100%
6.43
6.94
6.80
6.62
0.24
0.32
0.39
0.37
Campus
unmarked
Passenger Car
SUV/Van
Pickup (PU)
All Combined
77
32
35
144
53%
22%
24%
100%
5.90
6.15
6.37
6.07
0.43
0.42
0.33
0.45
Adjustment Factor
As previously mentioned, four people measured 50 vehicles to obtain distances from the tire
edge to the edge of the main vehicle body and to the edge of the left-side mirror. Since the tires
of two vehicles actually protruded past the edge of the vehicle body, the measurements from the
tire edge to the main body edge ranged from -0.125 to 0.25 ft (-0.038 to 0.076 m). For passenger
cars only, most measured no more than 0.2 ft (0.06 m) from the tire to the body edge.
For all vehicles, dimensions from the tire to the edge of the side mirror ranged from 0.25
ft (0.076 m) to 1.0 ft (0.305 m). Few passenger car side mirrors extended more than 0.65 ft (0.2
m) past the tire edge. The values in the 80th to the 90th percentile range (which did include
passenger cars) were slightly above 0.65 ft (0.2 m).
Vehicles seldom park perfectly parallel to the curb edge alignment, so it is common to
observe a slight skew in a parked vehicle’s position. Therefore, the protrusion of a side mirror
past the tire edge may effectively be less than indicated by the above measurements. Unless the
vehicle is parked at an extreme skew to the curb edge, this difference will be slight.
To reflect the additional width (i.e., past the tire edge) occupied by the vehicle body and
the mirror, adjustment factors of 0.13 ft (0.04 m) and 0.52 ft (0.16 m) were added respectively to
the measured offset width. These adjustments were included in cumulative plots of the data.
RESULTS
Table 4 presents, for each of the four area types, the number of vehicles measured and the
minimum, average, and maximum widths observed.
Gattis, Dammalapati, Cotton, Cotton
9
TABLE 4 Descriptive Statistics of Parked Vehicles
Area Type
Count
Mean
offset
(ft)
Standard
deviation
(ft)
Minimum Maximum
offset
offset
(ft)
(ft)
Commercial marked
Mobile
Fayetteville
278
88
190
6.88
6.81
6.92
0.61
0.57
0.64
5.3
5.3
5.6
9.7
9.2
9.7
Residential unmarked
107
6.21
0.53
5.3
8.5
71
6.62
0.37
5.8
7.5
144
6.07
0.45
5.0
7.0
Campus marked
Campus unmarked
Offset and Street or Traveled Lane Width
The data were examined to determine if either narrower streets or narrower traveled lanes were
associated with vehicles being parked closer to the curb. Among the area types in the study, the
unmarked residential streets exhibited the greatest range of street widths. Examining the data in
Table 5, there is no apparent trend between the measured offset values and width. Vehicles
parked on the more narrow residential streets had measured offsets similar to those on wider
streets. Likewise, no great differences were observed between offsets on a range of the wider
and a range of the narrower traveled-lane widths. In fact, vehicles on both the wider streets and
along the wider traveled lanes were positioned slightly closer to the curb. This could be a
response by those parking their vehicles on the wider streets to the more typical volumes and
speeds on the wider streets that were in the study (the narrow streets in the study have very low
volumes and speeds).
A linear regression analysis on the unmarked residential parking offset widths was
performed, with the moving lane width as the independent variable and the parking offset width
as the dependent variable. For residential area parking on streets with moving lane widths that
ranged from 8.8 to 15.8 ft (2.68 to 4.82 m), the computed R2 value was 0.01.
TABLE 5 Residential Parking Offset-to-Tire and Street Width
Widths (ft)
Low
High
Entire Street Width
30.1 to 40.0
21.9 to 26.5
19.1 to 20.3
19.1 to 40.0 (all)
Unmarked Residential area offset to tire
N
avg.
Min to Max
85th%
(ft)
(ft)
(ft)
32
40
35
107
6.1
6.2
6.3
6.2
5.3
5.3
5.3
5.3
to
to
to
to
7.1
8.0
8.0
6.6
6.5
6.6
7.0
6.6
Moving Lane Width (comparing “wide” with “narrow”)
Wide – 12.6 to 15.8
27
6.3
5.4 to 8.0
6.6
Narrow – 8.8 to 9.5
23
6.4
5.6 to 8.5
6.8
Gattis, Dammalapati, Cotton, Cotton
10
Examination of the data revealed that pickup trucks were overrepresented in the upper
range of offset values. This was observed in both the commercial and in the residential data.
Cumulative Distribution Plots
A review of a cumulative distribution graph can help identify patterns in and the outliers in a set
of data. In the built world in which we live, it is not uncommon to design for all but the outliers.
Figure 1 is a cumulative plot of the commercial area parking offsets, and Figure 2 shows
the distribution of the residential area parking offsets. An inspection of the plot from the
marked, commercial parking area shows the shape of the plotted line for the vehicle body does
not break until around the 97th percentile value, about 8.0 ft (2.43 m). With additional width for
the side mirror, this increases to about 8.4 ft (2.56 m). The break for the vehicle body in an
unmarked, residential setting is not as well defined, and occurs somewhere at or above the 90th
percentile value, perhaps between or 6.8 to 7.3 ft (2.07 to 2.23 m). Considering the side mirror,
this value is about 7.7 ft (2.35 m).
The plots of the campus-marked and campus-unmarked data did not show clear breaks.
The 90th percentile values were about 7.3 ft (2.23 m) for marked spaces and 6.7 (2.04 m) for
unmarked parking. With the additional width for the mirror, these values increase to 7.7 (2.35
m) and 7.1 ft (2.16 m).
DISCUSSION
The offsets from the curb face to the side of the tire facing moving traffic were measured for
vehicles parked in commercial areas with marked spaces, residential areas with unmarked
spaces, and a campus area with both marked and unmarked spaces. To arrive at a total parkedvehicle offset from the curb, and additional width of vehicle that extends past the tire was added
to the dimension measured. The following points are noted.
Χ
In both the commercial and the residential areas, pickup trucks extended over 0.25 ft
(0.081 m) farther out into the street than did passenger cars. A disproportionate number
of pickups were observed in the upper range of the parking offset values.
Χ
Over the ranges of street widths and traveled lanes of the residential streets in this study,
neither street width nor traveled lane width seemed to affect the parking offset width.
Χ
For all vehicles, the average offset of those parked in the marked commercial areas, 6.9 ft
(2.1 m), extended further into the street than did the average offset of those parked in the
unmarked residential areas, 6.2 ft (1.9 m).
Χ
The cumulative plots of parking offset widths suggest that in order to accommodate
approximately 90% of the parked vehicles in this study, the marked commercial spaces
would need to be about 8.0 ft (2.4 m) wide, and both the residential and the campus
unmarked spaces about 7.2 ft (2.2 m) wide. Note that these dimensions do not provide
any width to accommodate opening vehicle doors on the side of moving traffic.
There are a number of possible explanations of why vehicles parked in the marked
commercial spaces occupied a greater width than did those parked in the unmarked residential
areas. Two of the following possible explanations were offered by persons who are not authors.
1.
The difference was due to random chance.
2.
The difference was due to unidentified factors peculiar to the area.
Gattis, Dammalapati, Cotton, Cotton
11
CUMULATIVE PLOT - COMMERCIAL
Car Body
Car Mirror
100%
Rank (%)
80%
60%
40%
20%
0%
5.0
6.0
7.0
8.0
9.0
10.0
11.0
Max. Distance Curb Face to Car Body (ft)
FIGURE 1 Commercial parking offset width
CUMULATIVE PLOT - RESIDENTIAL
Car Body
Car Mirror
100%
Rank (%)
80%
60%
40%
20%
0%
5.0
6.0
7.0
8.0
9.0
10.0
11.0
Max. Distance Curb Face to Car Body (ft)
FIGURE 2 Residential parking offset width
Gattis, Dammalapati, Cotton, Cotton
FIGURE 3 Campus marked offset width
FIGURE 4 Campus unmarked offset width
12
Gattis, Dammalapati, Cotton, Cotton
3.
4.
5.
6.
7.
13
The difference was due to the presence of parking meters and other objects close to the
commercial streets, which were not present along the residential streets.
The difference was due to a higher turnover rate in the commercial areas: drivers parking
for a shorter duration may expend less effort aligning their vehicles.
The difference was due to greater through lane vehicle volumes and/or speeds in the
commercial area. Once they initially enter a parking space, drivers parking their vehicles
in the commercial area were less prone to make repeated maneuvers to better position
their vehicles, lest they strike passing through vehicles. But traffic volumes and speeds in
the study environments may have been low enough so that this was not a factor.
The difference reflects a “use all that is available” mentality. Since a marked 8 ft (2.4 m)
width was often present, motorists making the parking maneuvers tended to use all of the
space provided. If the marked spaces had been narrower, perhaps they would have
positioned themselves closer to the curb.
The difference reflects the difficulty in making a parallel parking maneuver as opposed to
pulling over to the curb. In the areas studied, only a small percentage of the residential
area curbsides are occupied by parked vehicles, so parking maneuvers usually consist of
simply aiming the vehicle toward the curb, and then aligning the vehicle parallel to the
curb while coming to a stop. In contrast, almost all of the commercial area parking
spaces are typically occupied during the day, so parking in the marked spaces in the
commercial area is much more likely to involve parallel parking maneuvers. Given the
greater difficulty of the parallel parking maneuver, it is not surprising that vehicles
parked in the marked spaces in the commercial area were positioned farther from the curb
than those parked in the residential area. To test this possibility, parking offsets would
have to be measured in an unmarked area where curb space is occupied to the extent that
drivers usually have to make a parallel parking maneuver.
CONCLUSION
At the beginning of this study, the possibility that the influence of narrow streets would cause
drivers to park their vehicles closer to the curb, and therefore take less width than called for in
traditional parking width guidelines, was recognized. However, the measured data from the
streets in this study do not support this hypothesis.
An urban street cross section that provided 7.0 ft (2.1 m) or less of parking width would
not accommodate 20% or more of the vehicles parallel parked along the curb. The traditional 8.0
ft (2.4 m) width did not appear to be excessive for the commercial area parking observed in this
study. A minimum width of at least 7.2 ft (2.2 m) would accommodate most of those parked in
unmarked residential areas. These dimensions do not include any width to accommodate
opening vehicle doors on the side of moving traffic; to accommodate this need, it could be
argued that additional width is needed along streets with higher volumes or speeds.
ACKNOWLEDGMENT
A special thank you goes to Joe Ruffer and James Foster of Mobile County Alabama, for
providing parking width data. The support of the Mack-Blackwell National Rural Transportation
Study Center at the University of Arkansas (through a grant from the U. S. Department of
Transportation University Centers Program) made this research possible. The views expressed
herein are those of the authors alone.
Gattis, Dammalapati, Cotton, Cotton
14
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