More evidence that wider roads encourage speeding

By Chris McCahill

Wider lanes and shoulders encourage faster driving, according to a new study published in the Journal of Transportation Engineering. Researchers from Texas A&M studied uncongested freeways in Dallas, Houston, and San Antonio. Based on more than 650,000 observations, they found that drivers travel 2.2 mph faster, on average, in 12-foot lanes than in comparable 11-foot lanes. Perhaps even more striking, wide left shoulders adjacent to 11-foot lanes can increase speeds by as much as 1.1 mph per foot of shoulder width, ranging from 1.5 to 11 feet.

As the authors note, these effects are more significant than those described in the Highway Capacity Manual, particularly with regard to left shoulders, for which there isn’t much guidance.

Unfortunately, the study also highlights how speed and capacity are often conflated in misleading ways and how safety can be ignored altogether. For example, as roads approach congestion, speeds don’t change much but higher speeds drop more quickly. Moreover, maximum flow typically occurs at speeds well below the free flow speed. Finally, past research shows that as density increases, high speeds pose a serious safety risk. Therefore, lower, more uniform speeds should be encouraged.

While 12-foot lanes are a common standard (and a minimum requirement in some states), the AASHTO Green Book and its companion, A Guide for Achieving Flexibility in Highway Design, support the use of narrower lanes in many circumstances.

Chris McCahill is a Senior Associate at SSTI.