85% of motorists drove over limit on I-310, study says

85% of motorists drove over limit on I-310, study says

The newly posted 70 mph speed limit on the elevated portions of Interstate 310 in St. Charles Parish marks a statewide trend aligning motorists’ apparent need for speed, but it’s also considered safer.

When a speed study in the area by the Louisiana Department of Transportation and Development (DOTD) showed at least 85 percent of motorists were driving at a speed higher than the one posted, the limit was raised from 60 mph, said DOTD spokesman Rodney Mallett.

“Speed limits are set at the 85th percentile because the majority of drivers are moving at that speed and studies show that going much faster or slower than the 85th percentile speed increases the chances of a crash,” Mallett said. “Therefore, the results of the study recommend the speed limit be increased to 70.

“It happens a lot. It just happened at I-12 and I-10, both in Baton Rouge, where the interstate was widened and that’s what the people were doing.”

Speed limits are also rising nationally.

They have risen on federal highways, some 70 mph or even higher to 75 mph. Many of the states that increased speed limits  are reporting a spike in crashes.

Speed limits on state roads are based on the results of engineering and traffic investigations, Mallett said.Roadway characteristics and shoulder conditions, such as curves, striping, surface width and type; grade alignment and visibility; roadside development factors, such as sites along the location which generate traffic; safe speed for curves or hazardous locations; parking practices and pedestrian activity; traffic volume; crash history; traffic signals and a speed study all help decide a speed limit.

Mallett said the speed study is the most important part of the traffic investigation.

A speed study was conducted in response to the request from St. Charles Parish from the I- 10 access ramp to the Hale Boggs Memorial Bridge on I-310.

In 1997, the Louisiana Legislature raised speed limits on all interstate routes from 65 to 70 mph, Mallett said. In 1998, the DOTD decreased the limit to 60 mph on the elevated sections of the interstate system due to the recommendations of a multi-agency study, entitled “Fog Related Accidents.”

The study was conducted as a result of high profile crashes that occurred on I-10 in 1996 and noted the absence of weather warning systems for motorists at the time. The study recommended an interim reduction of the posted speed limit to 60 mph on a number of elevated sections of the interstate system until safety enhancements were installed.

Mallett said, since that time, safety enhancements have been added to the elevated portions of I-310. Mile marker signs have been installed every 1/10th of a mile and reflective raised pavement markers placed along the roadway centerline.

In addition, a programmable Dynamic Message Sign was added to the I-310 interchange at U.S. 61 (Airline Highway).  The message sign is operated by the Regional Transportation Management Center, which is continuously staffed 24 hours per day, 7 days per week.

St. Charles Parish Councilman Paul Hogan, who prompted the speed study on the elevated portion of I-310, maintained the lower limit constituted a “speed trap” because it was inconsistent with other interstates where the limit is 70 mph.

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