If you spend much time on I-5, you’ll be interested in this: ODOT engineers are recommending that Oregon raise the freeway speed limit for trucks from 55 to 60 mph, but they expect this would increase actual truck speeds only a little if at all.
The state transportation department spent the winter and spring studying truck speed limits on freeway segments where there’s a difference between those and the speed limits for cars. That includes most of Interstate 5 in western Oregon.
On Thursday, June 8, the state’s Speed Zone Review Panel will meet in Salem to consider the final report of an engineering investigation and decide what to recommend to the Oregon Transportation Commission.
In the report, the state traffic/roadway engineer recommends raising the truck limit to 60 mph on all the freeway segments where the limit now is 55 mph for trucks and 65 mph for cars. One exception: In Roseburg, the report recommends keeping the limit at 55 for trucks and lowering it to 60 mph for cars, the same as on other urban segments of I-5.
The ODOT engineers found that trucks already travel at an average speed near 60 mph, and that 85 percent of trucks go 64 mph or less.
Says the report: “Numerous studies have found that establishing speed limits at a point between the average speeds and the 85 percentile speeds has resulted in the lowest crash rates.” The 85th percentile is the speed at or below which 85 percent of vehicles travel.
Even though the difference between current truck and car speed limits is 10 mph, the actual difference on the road is only 2-3 mph in average speeds and 4-5 mph in 85 percentile speeds, according to the report.
“The recommended increase in truck posted speeds (60 mph) would more closely represent the speeds trucks are actually traveling, i.e., the reasonable speed as determined by the drivers,” the report says. “This may reduce speed variance between all vehicles, which may further reduce conflicts and crashes.”
ODOT explains why it does not expect commercial truck drivers, unlike car drivers, to exceed the higher limit by much: Greater caution for fear of getting tickets and losing their jobs; speed governors in their rigs; a desire to save fuel; and a reluctance to pass cars and thus becoming “more conspicuous to enforcement.”
A table in the report shows that the Albany segment of I-5, between Coburg and South Salem, has only one State Police trooper available from 6 a.m. until 2 a.m. The 85 percentile speed is 70 mph for cars and 61 mph for trucks. With nearly 50,000 vehicles a day (in 2015), the segment also is among the most congested. Without the congestion, speeds might be higher yet. (hh)