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An Evaluation of Speed Cushions on Neighborhood Streets:
Balancing Emergency Vehicle Mobility with Traffic Calming Needs
Kevin Chang, Ph.D., P.E. and Matthew Nolan, P.E.
INTRODUCTION
Local jurisdictions frequently respond to public concerns about speeding on neighborhood
streets. When a speed study confirms that a significant percentage of vehicles are exceeding the
posted speed limit, the traffic engineer carefully reviews roadway conditions to determine if
additional safety measures need to be implemented. Preserving roadway safety for both the
motorized and non-motorized public alike who share the roadway is essential.
Each jurisdiction is likely to employ a number of solutions from its traffic safety toolbox.
Additional signing, use of mobile radar speed display units, neighborhood speed watch
programs, and targeted police enforcement may discourage drivers from traveling at
unacceptable speeds. Physical devices, such as traffic circles, speed humps, and speed tables can
also be considered, but will likely impact emergency vehicles by increasing their response times.
To balance increased driver awareness of travel speeds with emergency vehicle accessibility,
King County Department of Transportation installed a set of ten speed cushions along 36th Place
South / 39th Avenue South / South 294th Street / 45th Place South between South 288th Street and
South 298th Street as part of a pilot project. To evaluate the effectiveness of this traffic calming
device, speed and volume studies were conducted before and after installation.
The information presented here serves as a companion document to the presentation that was
given at the 2006 ITE Annual Meeting and Exhibit, in Milwaukee, Wisconsin.
BACKGROUND
This pilot project was carried out on 36th Place South / 39th Avenue South / South 294th Street /
45th Place South, an urban, north-south, neighborhood collector (see Figure 1). The width of this
roadway, 38 feet from vertical curb to vertical curb, provides adequate space for one travel lane
in each direction and on-street parking on both sides. Pedestrians and other non-motorized users
must share this space since neither sidewalks nor designated shoulder areas exist. There are no
lane markings on this neighborhood roadway. (As a general guidance, the King County
Department of Transportation does not place edgelines and / or centerlines on residential roads
unless there are compelling reasons to do so. To maintain a "quiet, non-arterial appearance",
pavement markings are usually reserved for roadways where speeds and volumes are higher.)
Prior to the installation of speed cushions, volumes on this neighborhood collector ranged from
over 900 vehicles per day (both directions) on the north end to nearly 1,800 vehicles at the south
end. Average vehicle speeds ranged from 24.2 miles per hour to 28.2 miles per hour on a
roadway posted at 25 miles per hour.
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FIGURE 1 : PHOTO OF 36th PLACE SOUTH / 39th AVENUE SOUTH / SOUTH 294th STREET / 45th PLACE SOUTH
(BEFORE)
In response to long-standing neighborhood concerns regarding speeding, reckless driving, and
side-swiping of on-street parked vehicles, staff from the King County Department of
Transportation’s Traffic Engineering Section held a series of public meetings and led public
outreach efforts to discuss alternatives to improving traffic safety. For this neighborhood, there
was strong consensus that the installation of preventative traffic calming devices be under
consideration, although staff explained that existing speed and volume data indicated that
roadway conditions were similar to those of comparable neighborhood collectors. Staff
members recognized, however, that the candidate roadway, with long sight lines in places,
manageable traffic volumes, and documented cases of reckless driving, could serve as a test
location for the implementation of speed cushions. As a result, King County created a pilot
project at this location.
DESIGN DEVELOPMENT
To determine an appropriate speed cushion design, staff from the King County Department of
Transportation conducted on-site investigations of similar devices in neighboring jurisdictions,
discussed possible design features with local fire department personnel, and reviewed existing
data and design standards. Special consideration was given to the different track and vehicle
widths of passenger and emergency vehicles.
As a reference for practitioners, Tables 1 and 2 provide information on the different spacing of
passenger and emergency vehicles that are commonly in use today on nationwide roadways.
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TABLE 1 : TRACK WIDTH AND VEHICLE WIDTH OF MOST POPULAR VEHICLES IN THE U.S.
Vehicle
# Sold
Track Width Vehicle Width
1
Ford F-Series
901,463
67.0”
78.9”
2
Chevrolet Silverado
705,891
65.0”-66.0”
78.5”
3
Toyota Camry
433,703
60.4”-60.8”
70.7”
4
Dodge Ram
400,543
67.9”-68.6”
79.5”
5
Honda Accord
369,293
61.1”-61.2”
71.3”
6
Honda Civic
308,415
59.0”-60.2”
68.9”
7
Nissan Altima
255,371
61.0”-61.2”
70.4”
8
Chevrolet Impala
246,481
61.5”-62.4”
72.9”
9
Chevrolet Malibu
245,861
59.3”-60.0”
69.9”
10
Chevrolet TrailBlazer
244,150
62.1”-63.1”
74.7”
SOURCE : J.D. POWER AND ASSOCIATES, AUTOMOTIVE NEWS DATA CENTER, 2005
TABLE 2 : TRACK WIDTH AND VEHICLE WIDTH OF SELECTED LIFE SAFETY VEHICLES
Ambulance Manufacturer
Track Width
Vehicle Width
Lifeline
72.0"-75.2"
92.8"-96.0"
Horton
n / a
94.3"-96.3"
Braun
n / a
96.0"
Road Rescue
n / a
90.0"-96.0"
Demers
74.5"-78.8"
94.0"
Fire Engine Manufacturer
Track Width
Vehicle Width
H & W
73.8"-82.8"
96"
Pierce Manufacturing
n / a
96"-101"
SOURCE : VARIOUS COMPANY WEB-SITES (SEE REFERENCE LIST FOR MORE INFORMATION)
Based on this review, the King County Department of Transportation created a design in which
the center speed cushion was 84 inches wide. The adjacent cushions to the left and to the right of
this center cushion were spaced 18 inches away. The intent of this design was to allow most
emergency vehicles to straddle the center cushion and drive through each set of speed cushions
without having to noticeably slow down, and to prevent most passenger cars and trucks from
making the same maneuver. Each speed cushion, patterned after the shape of a traditional speed
hump, was designed to be 3 inches in height and 14 feet in length. The final design and layout of
the King County speed cushion is shown in Figures 2 and 3.
To prevent drivers from reaching undesirable speeds between each speed cushion, engineers
conducted a careful field review to determine the best placement of each speed cushion along
this roadway. Field engineers wanted to balance effectiveness without having to install an
excess number of devices. Since 36th Place South / 39th Avenue South / South 294th Street / 45th
Place South was nearly one mile in length, a total of ten speed cushions were installed, with the
spacing between each set of cushions ranging from 366 feet to 761 feet. The placement of each
cushion was dependent on field conditions, including, but not limited to, driveway locations,
proximity to intersections, and sight distance concerns.
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FIGURE 2 : SPEED CUSHION LAYOUT (PLAN VIEW)
FIGURE 3 : SPEED CUSHION SPECIFICATIONS (PLAN VIEW)
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CONSTRUCTION SPECIFICATIONS
Each speed cushion was constructed using Class B, asphalt concrete pavement. The cushion was
rolled and then hand-tamped using a plate compactor by King County Road Maintenance
Division staff. The edges were subsequently crack-sealed to minimize deterioration over time.
To provide guidance to drivers, a white arrow was painted on the roadway surface in advance of
the speed cushion. Raised pavement markers were installed on top of the center cushion to
provide additional guidance. Appropriate warning signs were also installed at each location.
Figure 4 shows one of the speed cushion installations with signage and pavement markings after
construction.
FIGURE 4 : PHOTO OF 36th PLACE SOUTH / 39th AVENUE SOUTH / SOUTH 294th STREET / 45th PLACE SOUTH
(AFTER)
RESULTS
To evaluate the effectiveness of these traffic calming devices, volume and speed counts were
conducted at two different locations along this corridor before and after the installation of the
speed cushions (see Figure 5). Traditional rubber hose technology was used to collect this data
over a three-weekday period. For the after condition, special attention was given to place the
counters at the midpoint location between two sets of speed cushions to minimize the influences
of slowing by drivers either approaching or departing from a nearby device.
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FIGURE 5 : LOCATIONS OF TRAFFIC SPEED AND VOLUME COUNTS
Traffic speed and volume counts were collected in September 2002, June 2003, and May 2004
prior to installation of the speed cushions. Count data were collected after installation in October
2004 and April 2005. The results indicate that, in general terms, average speeds and volumes at
both locations decreased after installation of these traffic calming devices. A summary of the
before and after data is shown in Table 3.
TABLE 3 : SPEED AND VOLUME DATA – (BEFORE AND AFTER)
39th Place South, south of South 294th Street
northbound
Southbound
AVG
STDEV
VOL
AVG
STDEV
VOL
Jun-03
24.18
5.47
1382
26.14
5.73
1319
May-04
24.60
5.05
1497
25.92
5.30
1424
Oct-04
21.02
5.39
1026
22.53
5.88
1002
Apr-05
22.49
5.54
993
22.04
5.69
957
45th Place South, south of South 290th Street
northbound
Southbound
AVG
STDEV
VOL
AVG
STDEV
VOL
Sep-02
27.70
6.81
1427
26.92
6.61
1422
Jun-03
27.72
5.95
1412
25.99
5.85
1359
May-04
28.20
5.80
1425
25.41
5.24
1386
Oct-04
20.65
5.64
1004
21.43
5.82
914
Apr-05
20.76
5.03
1028
21.68
5.15
1007
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CONCLUSIONS
Since installation of these speed cushion devices, staff at the King County Department of
Transportation has received favorable feedback from both the residents who live along 36th Place
South / 39th Avenue South / South 294th Street / 45th Place South and the local fire department
personnel.
A number of specific findings, discussed below, may be beneficial to practitioners who have
installed or are considering implementation of this device:
• The reduction in traffic volumes along 36th Place South / 39th Avenue South / South 294th
Street / 45th Place South indicates that the installation of speed cushions has caused some
dispersal of traffic to other neighborhood roadways (undesirable) and nearby arterial
roadways (desirable). Some drivers likely used this roadway as a “cut-through” route
before speed cushions were installed, and staff recognized that a redistribution of traffic
onto adjacent neighborhood roadways was possible. To address this concern,
homeowners who lived on these adjacent roadways were contacted prior to installation
and informed of the potential repercussions. King County then required that at least 60%
of all impacted homeowners within the impacted area – an area that included both the
homeowners who lived along 36th Place South / 39th Avenue South / South 294th Street /
45th Place South as well as those who did not - sign a petition in which they
acknowledged that they were aware of this project and agreed to the potential changes in
neighborhood traffic circulation.
• While the local Fire Department prefers that roadways remain clear of traffic calming
devices, the speed cushion design seems to have minimized the increase in response
times that are typically experienced by emergency services personnel when traveling over
or around traditional traffic calming devices. As a result, response from this area’s fire
department personnel has been generally positive. It should also be pointed out that this
route has not and does not function as a primary response route.
• King County is currently in the process of analyzing video footage to determine
compliance levels of this device in terms of whether or not a driver stays in his or her
travel lane. An initial review indicates that a majority of drivers are not intentionally
deviating from their normal travel path and straddling the center speed cushion in an
attempt to minimize the impacts of this physical device.
• Within the Puget Sound region, a number of local jurisdictions have begun experimenting
with speed cushions. Since there is not a “standard” regarding the design and associated
markings, there are presently many variations of the speed cushion in terms of shape,
materials used, color, and pavement markings currently in use on local roadways. To
minimize potential confusion for local drivers, development of a “regional standard”
should be considered as more speed cushions are likely to be installed in the future.
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REFERENCES
Braun Ambulances. http://www.braunambulances.com/
Demers Ambulances. www.demers-ambulances.com
H & W Emergency Vehicles. www.hwev.com
Horton Ambulance. http://www.hortonambulance.com/models.shtml
J.D. Power and Associates, Automotive News Data Center.
autos.msn.com/advice/article.aspx?contentid=4023245.
Lifeline Ambulance. www.lifelineambulance.com
Manual on Uniform Traffic Control Devices. U.S. Department of Transportation, Federal
Highway Administration, Washington, D.C., 2003.
Pierce Manufacturing. www.piercemfg.com
Road Rescue Ambulances. http://www.roadrescue.com/roadrescue/
CONTACT INFORMATION
Kevin Chang, Ph.D., P.E. (corresponding author)
Supervising Engineer, Neighborhood Traffic and Pedestrian Safety Programs
King County Department of Transportation
201 South Jackson Street, mailstop KSC-TR-0222
Seattle, WA 98104
P (206) 263-6131; F (206) 296-0176
kevin.chang@metrokc.gov
Matthew Nolan, P.E.
County Traffic Engineer
King County Department of Transportation
201 South Jackson Street, mailstop KSC-TR-0222
Seattle, WA 98104
P (206) 296-3761; F (206) 296-0176
matthew.nolan@metrokc.gov