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Study: Higher Interstate Speed Limits are Safe
Purdue University study concludes raising the interstate speed limit in Indiana had no negative safety consequence.

Fred Mannering
Purdue University this week released results of a study showing that there was no change in the number of accidents after Indiana increased the maximum freeway speed limit to 70 MPH on July 1, 2005. Civil engineering Professor Fred Mannering led the team that looked at accident data from one year before this change -- when the top legal speed was 65 MPH in rural areas -- for comparison with accident rates a year later.

"Everybody expects that when you increase the speed limit, injuries and the severity of injuries are going to increase, but that hasn't happened on the interstate highway system in Indiana," lead researcher Fred Mannering said in a statement.

Mannering's study noted that expert opinion is divided on this controversial subject. For example, a 1999 report sponsored by the Insurance Institute for Highway Safety claimed increased limits resulted in higher accident rates. The insurance industry depends on speeding tickets to provide surcharge revenue. Other independent studies, including a 1994 review of the effect of the change from the national 55 limit to 65 on rural roads, have arrived at a contrary conclusion that the higher limit, in fact, saved lives.

Mannering used a statistical model to calculate accident probabilities based on his own examination of data from 390,000 accidents recorded by the Indiana Vehicle Crash Record System. After applying the model, he concluded that the increased speed limit had no effect on the probability of suffering an accident nor did it increase the severity of accidents that did occur.

In 2006, only 5.78 percent of the state's accidents were caused by unsafe speed, a decrease from 2004. The study also noted that a 15 MPH increase in the speed limit did not produce a 15 MPH increase in the actual speed traveled. Instead, real speeds increased only 12 MPH. The report suggested that speed limit changes may have had a negative impact on some non-interstate rural roads and that future changes for secondary roads should be evaluated on a "case-by-case basis."

An earlier version of the paper presented before the Transportation Research Board is available in a 140k PDF file at the source link below. The report will also appear in an upcoming issue of the Transportation Research Record.

Source: PDF File Analysis of the Effect of Speed Limit Increases on Accident-Injury Severities (Purdue University, 6/23/2008)

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