We hear that question often! And yes, these traffic calming devices are all slightly different. The appropriate device is selected for a roadway depending upon its geometry, the sight distance, and traffic characteristics such as speed and volume.
We use “speed bumps” in parking lots where speeds are 5-10 MPH. On our non-arterial streets we use what are called “speed humps,” and here’s the confusing part – they are commonly known as “speed bumps” by the public and are identified by signs as being speed “bumps” even though they aren’t. Speed humps extend the full width of a street. They are actually longer (12’ long) and less aggressive (3” high mounds) than speed bumps, and are made to reduce speeds on our non-arterial residential streets. They are not suitable for arterials streets or on all non-arterial residential streets.
“Speed cushions” are modified speed humps which we use on low volume arterial streets and on non-arterials streets that have been designated as an emergency fire routes. How do they differ from bumps and humps? Well, they are basically a modified speed hump that has been divided into sections to allow vehicles with a larger wheel bases, such as a fire truck or ambulance, to “straddle” them without delaying their response time. Another benefit of the speed cushion for emergency response vehicles is that there isn’t a sudden jolt to the vehicles which can be harmful to the sometimes delicate equipment they are carrying. Buses can also travel over these speed cushions without jostling the passengers.
For more information about the traffic calming devices we use please visit our web page at http://www.seattle.gov/transportation/ntcp_physical.htm