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."
Comments
Confusion About Narrow Lanes
The entire article is about reconfiguring expressways so they can have more lanes in the same road width. Then, at the very end, they suddenly change their story and say:
"They require smaller structures and less earth moving. Neighborhoods suffer less disruption, an advantage accentuated by the lower free-flow speeds. Therefore, urban residents are likely to benefit from the smaller environmental footprint of these roads as well as from their superior ability to carry high-peak traffic flows."
This conclusion would be true if they had been talking about building the same number of lanes in a narrower space. But since they have been talking about building more lanes in the same space, it is patently false that the roads would have a "smaller environmental footprint." The roads would require structures that are just as large and would require just as much earth moving.
Because of "their superior ability to carry high-peak traffic flows," they would generate induced demand, increasing the road's environmental footprint.
More, narrower lanes?
More lanes = less ped friendly, more car traffic. Drivers tend to drive slower in narrower lanes, but we should be adding bike lanes and transit, not more car lanes.