In 2006 Texas established the highest statutory speed limit in the United States when it raised the daytime speed limit for passenger vehicles on designated segments of I-10 and I-20 in west Texas from 75 to 80 mph. To assess the effect of the new limit, speeds of free-flowing traffic were measured at multiple sites on the segments with higher limits the month before and 3, 12, and 16 months after the limit was changed. To estimate what changes would have been expected without the increased limit, speeds also were measured on another, similarly designed highway in west Texas where the daytime speed limit remained at 75 mph throughout. Speeds were for free-flowing vehicles measured by photo radar. During the 16-month period following the speed limit increase, mean speeds of passenger vehicles on I-20 were estimated to increase by about 9 mph relative to the comparison road, where no speed limit change occurred and traffic speeds declined. On I-10 it was estimated that mean speeds of passenger vehicles during the 16-month study period increased by about 4 mph relative to the comparison road. Limiting the analysis to the month before the speed limit change and 1 year later, it was estimated that the proportion of drivers exceeding 80 mph was 18 times higher on I-20 and 2 times higher on I-10 as a result of the higher limit. The smaller speed increases on I-10 may have been related to its proximity to the US border with Mexico, which is heavily patrolled by law enforcement. Several highly visible border patrol initiatives coincided with posting of the higher speed limit. The I-20 sites did not have the same level of enforcement. Long-term monitoring of travel speeds in other states suggest that in future years traffic speeds on rural interstates in west Texas are likely to continue to increase. (Author/publisher)
Samenvatting