| The Federal Highway Administration reports evidence | | | | based mainly on speed limit and speed changes on |
| that crash risk increases for vehicles traveling much | | | | high-speed roads. More research is needed to assess |
| faster or slower than average and is lowest near the | | | | their applicability to low-speed urban roads. |
| average speed of traffic. The occurrence of a large | | | | In general, changing speed limits on low and moderate |
| number of crashes involving turning maneuver partly | | | | speed roads appears to have little or no effect on |
| explains the increased risk for motorists traveling | | | | speed and thus little or no effect on crashes. This |
| slower than average and confirms the importance of | | | | suggests that drivers travel at speeds they feel are |
| safety programs involving turn lanes, access control, | | | | reasonable and safe for the road and traffic |
| grade separation, and other measures to reduce | | | | regardless of the posted limit. However, on freeways |
| conflicts resulting from large differences in travel | | | | and other high-speed roads, speed limit increases |
| speeds. When the consequences of crashes are | | | | generally lead to higher speeds and crashes. The |
| taken into account, the risk of being involved in an | | | | change in speed is roughly one-fourth the change in |
| injury crash is lowest for vehicles that travel near the | | | | speed limit. Results from international studies suggest |
| median speed or slower and increases exponentially | | | | that for every 1 mi/h change in speed, injury accidents |
| for motorists traveling much faster. One of the major | | | | will change by 5 percent (3 percent for every 1km/h). |
| concerns in studies is the travel speed before the | | | | However there is limited evidence that suggests the |
| crash. Emerging technology used in mayday, vehicle | | | | net effect of speed limits may be positive on a |
| tracking, and adaptive speed control systems provide | | | | system wide basis. More research is needed to |
| the opportunity to accurately and continuously capture | | | | evaluate the net safety effect of speed limit changes. |
| travel speed. This technology should be applied in | | | | Most of the speed related crashes involve speed too |
| improving our understanding of the relationship | | | | fast for conditions. This would suggest that variable |
| between speed, speed variation, and safety. | | | | speed limits that adjust with traffic and environmental |
| When a crash occurs, its severity depends on the | | | | conditions could provide potential benefits. |
| change in speed of the vehicle at impact. The fatality | | | | Despite the large number of references concerning |
| risk increases with the change in speed to the fourth | | | | traffic calming, very few reports include results of a |
| power. International research indicates the change in | | | | systematic evaluation. In many cases traffic volumes |
| injury crashes will be twice the percentage change in | | | | as well as speed are reduced. As a result of the |
| speed squared, and fatal crashes will be four times the | | | | traffic diversion, crashes may be migrating to other |
| percentage change in speed. These relationships are | | | | roads. |