Developing Design Standards for Speed Cushions

Physical devices such as traffic circles and speed humps have long been considered possible solutions to speeding and other neighborhood traffic safety problems. The primary drawback has been their documented effect on fire and other life-safety vehicles caused by increased response times. To balance driver awareness of travel speeds with emergency vehicle accessibility, the King County, Washington, Department of Transportation designed, constructed, and evaluated a traffic-calming device referred to as a speed cushion. Speed cushions resemble speed humps but are constructed with channels to allow for minimal impact and delay to wider-wheel-based emergency vehicles such as fire engines. Ten speed cushions were installed along 36th Place South, 39th Avenue South, South 294th Street, and 45th Place South between South 288th Street and South 298th Street in unincorporated King County as part of a neighborhood traffic-calming pilot project. To evaluate the effectiveness of this traffic-calming device, speed and volume studies were conducted pre-and postinstallation. This report summarizes the results and findings from this study, which determined that speed cushions can provide traffic-calming benefits without significantly affecting emergency responders in fire and other life-safety vehicles. This report also discusses important next steps for researchers and practitioners needed because of limitations in the existing literature on this device in terms of material, color, shape, and roadway placement. The increased popularity of speed cushions has accelerated the need for standardization in terms of design and construction, and specific recommendations are provided in this report.