A six year study comparing accident frequency at nine ramp metering locations was conducted on the Superstition Freeway in Arizona. Three years of before and three years of after data were used to compare the change in the number of accidents both on the mainline and ramps. Ramp metering is only in operation during peak periods making it possible to also compare operations within periods of the day.
When ramp metering is in operation, rear-end and Sideswipe accidents were reduced 10 percent from the before case. However, during the periods when ramp metering was not in use accidents increased 33 percent. Accidents on the ramp increased in the after case due to traffic being required to stop on the ramp during metered periods. Also, accidents increased during periods when metering was not being used. Overall, accidents increased 24 percent on both the mainline and ramps when metering was in effect, and 43 percent when metering was not in effect. These increases may be due do large increases in traffic volume between the two cases.