Can Narrow Lanes Improve Traffic Congestion?
The U.S. Interstate Highway System has strict design standards that theoretically provide safe travel at high speeds, but only off-peak drivers have the opportunity to travel at high speed on these expressways. Small and Ng say, "But suppose we can design new roads, or reconfigure existing roads, to have lower speeds, neighborhood-friendly footprints, and higher capacity." For example, the standard 12-foot-wide lane is considered safe for mixed traffic at high speeds, but Small and Ng propose a road of the same total width with narrower lanes and shoulders. This would allow for more lanes to increase capacity, which means peak travelers can drive faster.
Small and Ng take a closer look at wide versus narrow lanes by comparing Expressway R of Interstate standards with Expressway C of a more compact design. They find that R has a higher free-flow speed (65.5 mph) than C (60.4 mph) under light traffic conditions, but as traffic volume increases, the decline in speed occurs more quickly on R with its fewer lanes. This was the same result when measuring average travel times. "Repeating these calculations at different traffic levels and peak-to-off-peak ratios," confirm Small and Ng, "we find the compact design performs better under all conditions in which there is appreciable queuing." The reason is that compact design depends on traffic volume, whereas regular design depends on free-flow speeds.
As for safety considerations, "it is uncertain whether the compact road design will reduce or increase safety." Wider lanes are generally regarded as safer because they provide leeway if drivers wander from their lane or swerve to avoid an accident, but there is no strong empirical evidence. "The most important factor is likely the speed chosen by drivers, which suggests a policy response," say Small and Ng. "reduce the speed limits on compact roads and add other measures to discourage speeding."