Case Study No. 26
Emergency Vehicles and Traffic Calming
Clark County, Washington
Prepared by Charles P. Green, Parsons Brinckerhoff,
Portland, Oregon.Information provided and contributions made by Charles P. Green, Parsons Brinckerhoff; Jennifer Green; Steve Green; Don Williams, Clark County; Gerald Morris, formerly
with Clark County Public Works;
Carl Switzer, Parsons Brinckerhoff.
Clark County needed traffic calming measures that would slow speeds on neighborhood streets, yet accommodate emergency response vehicles.
An emergency response speed hump.
NE 76th Street is a neighborhood collector (non-arterial classification) in Clark County, Washington, an unincorporated area outside of the City of Vancouver. The street is an eastern extension of an arterial roadway at Ward Road, and it connects two arterials, Ward and 162nd Avenue, at each end. The posted speed limit is 25 mi/h (40 km/h).
The street is a community place, a transportation facility not only for motor vehicles but for bicycles and pedestrians. Since the neighborhood street lacks continuous sidewalks and has no bike lanes or pathways, it was critical to improve pedestrian and bicyclist safety and comfort by maintaining slow vehicle speeds along the street. Children are frequently observed walking or riding on or across the street and to school (and school bus stops).
In 1997-98, Clark County approved and implemented a neighborhood traffic calming project for an approximate 1.2 mi segment of NE 76th Street between Ward Road and NE 162nd Avenue. The NE 76th Street project is innovative in that it has Clark County's first two tests of emergency vehicle-type traffic calming design. The first device, an emergency response speed bump, has a median and wheeltrack channel cut into the center of the bump to allow emergency vehicles to pass through the center, unimpeded, while general traffic is to legally slow down and use the bump. The second device, an emergency response traffic circle, has wheeltrack channels cut through the center of the traffic circle to also allow for emergency vehicle passage, while general traffic must travel around the circle. The circle's emergency vehicle channel is offset 15 degrees to discourage general vehicles from illegally shortcutting through the center of the device.
Prior to installation, these devices were tested in a closed-environment test as well as a field test. The closed-environment test was at the Clark County Maintenance and Operations facility, where a fire truck was used to test different wheeltrack and channel layouts using railroad ties. The spacing and median width specifications were developed from these tests.
Speed bump testing was also conducted in a field test by Clark County Public Works and Clark County Fire District 6 staff. A set of speed runs was made before and after regular speed bump installations on NE 129th Street in the Salmon Creek area. The result of the speed run indicated that a typical speed bump slowed fire trucks between 4-6 seconds per device (5 seconds per device, on average).
A closed-environment test was made using a similar fire truck at the County Maintenance yards. The results indicated that with the specified design wheeltrack/ median width, fire trucks should be slowed, at most, by 1-2 second per device to allow the driver to align and maneuver through the channel.
Prior to installation, a speed study was conducted in July 1996 by Clark County. The table below gives a summary of before and after traffic speeds with the two case study devices.
Before and after traffic speeds with the two case study devices.
|After Installation: Speed Bump
||After Installation: Traffic Circle
|Posted Speed (mi/h)
|Mean Speed (mi/h)
|85th Percentile Speed
|Pace Speed (mi/h)
Both types of devices slowed traffic speeds to match the neighborhood character and street designation, and allowed for emergency response vehicles to travel through them unimpeded. The devices have reduced speeding, thereby improving pedestrian and bicyclist safety and comfort. The only point of concern for bicycles and pedestrians appears to be the traffic circle. Some residents have remarked that the circle requires vehicles to maneuver around it, passing through what would be considered the pedestrian crosswalk. Additionally, some residents mentioned that the traffic circle appears to be an attractive "play area" for neighborhood children, which is a safety concern.
With the reduced speeds around the traffic circle and the improved around the device, there does not appear to be any evidence that the circle has increased conflicts between vehicles and pedestrians. The circle's design is similar to the design used in nearby Portland, Oregon and elsewhere, which also experience vehicle maneuvering in the crosswalk area. To date, there is no known data that would indicate that pedestrian safety is compromised by the circle's design.
The results of the testing are critical. Emergency services agencies generally set a response rate from time of call to time of arrival at the site varying from 3-6 minutes. Clark County adopted a policy on emergency response routes that traffic calming devices should not delay emergency response times by more than 30 seconds per emergency route. This policy was supported by the local emergency service providers. At a 5-second delay per speed bump, this allows for only 6 regular-design speed bumps to be installed on any given response route. This would essentially prohibit placing additional traffic calming devices on that route or on intersecting streets, as they would extend emergency response times beyond the desired 30-second threshold.
With the testing results shown above, a minimal delay of 1-2 sec per device over the length of the emergency response route was experienced. This allows for traffic calming devices to be installed on adjacent streets, or on an emergency response route, while still preserving emergency response times.
Public opinion, compared to the "after" results of the devices, seems to indicate that the county lacks an educational program to inform residents about the effectiveness of the devices. Some residents believe that speeding has not been controlled after the installation of these devices. While speeding has been shown to be significantly reduced, often below the posted speed limit, there is a prevailing perception amongst residents that the devices could be more effective.
Chuck Green, P.E.
Supervising Transportation Planner
Parsons Brinckerhoff Quade and Douglas (formerly with Clark County Department of Public Works)
400 SW Sixth Avenue, Suite 802, Portland, OR 97204
Phone: (503) 274-7223
Fax: (503) 274-1412