Appendix - Speed, Volume and Vehicle Collision Impacts
The speed, volume, and vehicle collision impacts listed in the tables below are based on hundreds of studies that are described in the ITE Traffic Calming: State of the Practice publication [1].

Table 4: Traffic Speed Impact

Traffic Calming Device Average percent change of
85th percentile speed 		 Notes
Bulb-outs
 		-4% See [2]
Chicanes
 		N/A
 		More reduction in top speeds versus average speed.
Chokers
 		-4% See [2]
Medians and Median Islands  -4% See [2]
Mini-Traffic Circles -11%
Mounds
 		N/A
Partial Street Closures -19%
Radar Speed Signs
 		N/A
Raised Crosswalks
-45%
N/A
 		For 22' speed tables
For longer tables
Roadway Narrowings -4% See [2]
Roundabouts N/A
Speed Humps
 		-22%
-23%
 		For 12' speed humps
For 14' speed humps


Table 5: Traffic Volume Impact

Traffic Calming Device
 		Average Percent Change of
Traffic Volume
 		Notes
Forced-Turn Channelization
 		N/A
Full Street Closures -44%
Medians and Median Islands
 		-10%
 		See [2]
Partial Street Closures -42%
Raised Crosswalks
 		-12%
Speed Humps
 		-18%
-22%
 		For 12' speed humps
For 14' speed humps
Turn Restriction Signs
 		N/A

Table 6: Vehicle Collision Impact

Traffic Calming Device
 		Average Percent Change of
Vehicle Collisions
 		Notes
Mini-Traffic Circles -71%
Raised Crosswalks
-18%
-9%
 		For 22' speed tables
For longer tables
Roundabouts -50% to -90% See [3]
Speed Humps
 		-13%
-40%
 		For 12' speed humps
For 14' speed humps

[1] Traffic Calming State-of-the Practice (Ewing, 1999)
[2] The speed, volume and collision impacts for each type of narrowing device (i.e., bulb-out, choker, medians, median island, lane narrowing) are not available by ITE and are instead listed as being the same across all the types of narrowing devices.
[3] At intersections previously controlled with traffic signals or stop signs. They can also handle 30% more traffic than intersections with signals and eliminate the need to widen roads to increase intersection capacity. [Dan Burden, Streets and Sidewalks, People and Cars – The Citizens’ Guide to Traffic Calming, page 42.]