MATT HELMS: Speed limit rises to match habits
BY MATT HELMS
FREE PRESS COLUMNIST
October 11, 2006
We knew it was coming. Now we know when.
Drivers will be able to travel at 70 m.p.h. on I-696 worry-free Nov. 9, the day the state has set to officially raise the top legal speed on Michigan's busiest freeway.
I-696 will join several freeways as recent members in the 70 m.p.h. club, including a large section of M-59 between Pontiac and Utica, I-75 between Outer Drive in Detroit and Pennsylvania in Taylor and much of the Van Dyke Expressway in Macomb County.
The Michigan State Police and the Michigan Department of Transportation have been raising limits on freeways around the state, saying that properly set limits encourage more uniform traffic speeds and, therefore, fewer crashes.
Police say drivers are more likely to obey realistic speed limits, so raising the limits reduces the speed differences between those who obey low limits and those who drive faster. Reducing the differences in turn decreases the likelihood that faster drivers will change lanes, brake suddenly, tailgate slower drivers or otherwise engage in behaviors that can lead to crashes, State Police say.
Rather than expecting faster traffic with the higher speed limit, "our experience shows us that the fastest traffic on the road might drive slightly slower when we post a more appropriate speed limit," says 1st Lt. Thad Peterson, commander of the State Police traffic services section.
An example is I-69 near Flint, which went from 55 to 70 m.p.h. in 2005.
Police measurements found the freeway's overall speeds decreased or largely stayed the same after the increase. The benefit: differences between faster and slower traffic decreased.
MDOT and the State Police recently concluded studies and found no structural or design issues on I-696 that would caution against raising the limit.
Scott Barrett of Macomb Township drives 45 minutes to an hour a day each way to work in Southfield and says he hopes the speed limit change will reduce his commute time.
"In my mind, if the speed limit's 70 all the way through, traffic will flow more smoothly the whole way," Barrett says.
Nov. 9 is also the day for new speed-limit rules on Michigan freeways. The minimum speed limit on freeways rises to 55 m.p.h., while the maximum truck speed limit rises from 55 to 60 on freeways where the overall maximum speed is 70.
Interstate 696, with 200,000 or more vehicles a day on some stretches, is the state's traffic-count king.
Other busy metro Detroit freeways, including portions of I-75 and I-94, also could see limits raised to 70.
Peterson says that most state freeways that aren't already at 70 will be studied in the next year or so to see whether a raised speed limit is in order.
The rising speed limits are based on a nationally followed engineering principle called the 85th percentile rule. It says speed limits should be set at the speed at or below which 85% of all traffic moves.
In other words, if 85% of all traffic goes 55 m.p.h. or slower, 55 is set as the maximum limit, and the 15% of drivers going faster than 55 are the exception -- and the drivers police are most likely to pull over.
Contact MATT HELMS at firstname.lastname@example.org -- e-mail is best -- or leave a message at 313-222-1450.