effects The of interstate speed limits on driving speeds: Some new evidence
by
Fred Mannering Professor of Civil Engineering Purdue University West Lafayette, IN 47907 [e-mail]
Abstract Since the repeal of the 55 mph national maximum speed limit on US interstate highways, speed limits have been raised across the country to as high as 75 mph and more. While many studies have addressed the effects of increased speeds on accident frequency and severity, the factors that determine drivers' choice of speed in the presence of speed limits are still not well understood. This paper provides additional insight into drivers' choice of speed by using a survey of Indiana drivers. The survey is timely since Indiana recently raised speed limits on rural interstates to 70 mph. With urban and suburban areas included, interstate speed limits in the state are now 55 mph, 65 mph and 70 mph. Using seemingly unrelated regression estimation, models of normal interstate driving speeds with low-traffic conditions are estimated for 55 mph, 65 mph and 70 mph speed limits. The results show a wide range of factors (gender, age, income, number of children, age driver is first licensed, assessment of pavement quality, and assessment of vehicle manufacturers) influence individuals' normal interstate driving speed and that the effect of these factors changes as the posted speed limit changes. Introduction Since the passage of the Emergency Highway Energy Conservation Act in 1974, which mandated the 55 mph national maximum speed limit on interstate highways in the US, the controversy relating to the effects that speed limits have on observed driving speeds and highway safety has been ongoing. This controversy has been fueled by various research findings and subsequent legislation, such as the National Highway System Designation Act of 1995 that gave states complete freedom to set interstate speed limits. As a result of such legislation, many states have raised rural interstate speed limits from 55 mph to 65 mph, 70 mph, or more. Although the numbers noticeably vary, almost all research efforts have concluded that the lower 55 mph speed limits have saved lives relative to the return to higher speed limits (1, 2). Some of the most recent work by Kockelman and Bottom (3) concluded that a speed limit increase from 55 to 65 mph resulted (on average) in roughly a 3% increase in the accident rate and a 24% increase in the probability of a fatality once an accident occurred. The Kockelman/Bottom study also estimated that speed limit increases from 65
to 75 mph resulted in a lower (relative to the 55 to 65 mph increase) 0.64% increase in the accident rate and in a lower 12% increase in the probability of fatal injury once an accident occurred. The authors speculated that the lower increases from 65 to 75 mph speed limits may have been the result of drivers' heightened awareness of risk at higher speeds or that roads assigned the higher 75 mph in their study's sample may have been inherently safer. However, not all studies are in agreement that the effect of higher speed limits has caused a decline in safety. One study contended that the legislation-enabled increase from 55 mph to 65 mph actually saved lives due to shifts in law enforcement resources, the ability of higher-speed-limit interstates to attract riskier drivers away from inherently more dangerous non-interstate highways, and possible reductions in speed variances ...
TRB_Paper_07-0120