Automated vehicles will bring big highway capacity increases

By Eric Sundquist

As the transportation field grapples with the impending impacts of automated vehicles, one AV-related outcome seems clear: Highway capacity will dramatically expand.

One new estimate comes from Dwight L. Farmer in the November ITE Journal (paywall). Because automatic braking systems react much faster than human drivers do, safe spacing on freeways can be reduced by about half. As a result, Farmer concludes (p. 36) that the current rule of thumb that a freeway lane can handle a flow of 2,000 vehicles per hour will be radically changed:

Based on this analysis and given the current state of autonomous vehicle technology development, it would appear that the impacts on our transportation system potentially include:

  • Increases in highway capacity by ~100 percent
  • Increases in expressway travel speeds by >20 percent
  • Significant reductions in accidents and injuries

An even more aggressive estimate, taking into account the possibility of AVs platooning in a “hypothetical continuous train,” could raise capacity even further, to an astronomical (by today’s standards) 8,000 vehicles per hour at 60 mph, or 10,000 vehicles per hour at 80 mph.

While acknowledging that many questions remain, e.g. around a fleet that is partially autonomous, Farmer concludes that “it appears that the evolution of autonomous vehicle technologies will continue to require the rewriting of the fundamentals of traffic flow, travel mobility, as well as travel demand forecasting.”

The obvious policy response would be to err on the conservative side in planning new highway capacity. New lanes being planned now and delivered at a time that AVs dominate the fleet may not only waste resources that could be better used, but also induce more driving, potentially leading to a dystopian AV future.

At least one major transportation department has done as Farmer suggests and adjusted its modeling to account for the coming effect of AVs: the Contra Costa County (Calif.) Transportation Authority, which manages the Bay Area county’s sales-tax funded transportation program. During an October presentation, Executive Director Randy Iwasaki said that his agency was now using 3,200 as a rule of thumb for vehicles per hour per lane for freeways in planning—a figure that is substantially higher than the conventional 2,000 but far lower than Farmer’s near-term estimate of 4,000. (Iwasaki also mentioned narrower lanes and shoulder-running as other potential AV-related strategies.)

Do you know of other cases of DOTs or other transportation agencies adapting policy and practice to address AVs? If so, please share by e-mailing Robbie Webber at

Eric Sundquist is Managing Director of SSTI.