PennDOT: Increase to 70 mph improved speed limit compliance

By Tyson Fisher, Land Line staff writer | 8/9/2016

Drivers are more likely to comply with the posted speed limit after the increase from 65 mph to 70 mph, a Pennsylvania Department of Transportation study suggests.

In July and August 2014, PennDOT increased speed limits on rural sections of Interstates 80, 380 and 76 from 65 mph to 70 mph. To determine whether or not speed limit increases should be implemented on other sections of the interstates, PennDOT analyzed the effects of the pilot rural sections, including crash rates and speed compliance.

One set of data measured the average speeds before the speed limit increase and approximately two to four months after. When the limit was at 65 mph, the mean speed for passenger vehicles was 69.7 mph. Two to four months after the switch to 70 mph, the average speed increased by only 1.4 mph to 71.1 mph. For heavy trucks, there was a 2.3 mph increase to 67.1 mph.

Over a longer period of time, average speeds decreased. Data collected nine to 10 months after the speed limit increase revealed only a 0.1 mph increase for passenger cars when compared to data collected two to four months after the change. However, heavy truck average speeds decreased 0.3 mph from 67.1 mph to 66.8 mph.

Comparing average speeds from before the speed limit increases to nine months after revealed optimistic results regarding compliance. Heavy truck average speeds increased by only 2 mph, with passenger car speeds increasing 1.5 mph despite a 5 mph limit increase. Accounting for all vehicle types, average speeds increased by only 0.5 mph.

Speeds at the 85th-percentile of drivers also showed promising results. Among all vehicle types, 85th-percentile speeds from before the speed limit change to 9-10 months after increased by only 0.7 mph. Truck 85th-percentile speeds increased by 2.8 mph, and passenger car speeds went up by 1.2 mph.

More than 80 percent of passenger cars and nearly 50 percent of trucks exceeded the posted speed limit when it was set at 65 mph. When the limit increased to 70 mph, that number dropped to nearly 50 percent of passenger cars and nearly 13 percent of trucks within two to four months after the change. However, once drivers grew accustomed to the new speed limit, the numbers started to go back up. Approximately 9-10 months after the speed limit increase, 39.45 percent of passenger cars and 21.3 percent of trucks were driving over the limit, still significantly down from 80 percent and 50 percent, respectively.

According to a PennDOT spokesperson Rich Kirkpatrick, no significant crash data jumped out regarding the speed limit change. PennDOT will continue to monitor the pilot areas while considering expansion of the speed limit increase to other areas. The agency will wait until Spring 2017 to look at the updated crash data for the 70 mph sections before deciding on any expansion, Kirkpatrick told Land Line.

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