The speed limit for trucks on sections of Oregon's western freeways is set to increase from 55 to 60 mph by the end of the year.

The Oregon Transportation Commission approved the increase last month. The increase will take effect when new speed signs are posted, work that is expected to be finished in December.

The change affects Interstate 5 south of Portland, except where it passes through urban areas; Interstate 205 from the junction of I-5 to West Linn; and I-84 from Troutdale to The Dalles.

State transportation officials found that truck drivers were generally traveling faster than posted speeds already, averaging 60 mph.

They noted, citing a report from Portland State University, that Oregon is one of only seven states that previously posted lower speed limits for cars and trucks, and that there's no evidence that differential speed limits are safer or less safe.

That report also noted, however, that increasing speeds is expected to lead to more crashes.

The Oregon Transportation Commission unanimously voted in favor of the increase. Commissioner Sean O'Hollaren said he believed reducing the differential in speed between cars and trucks would improve the flow of traffic.

Commissioner Dave Lohman said his was a reluctant "yes" vote because he would prefer the speed limit for cars be lowered to match that of trucks.

Meanwhile, the speed limit for passenger vehicles on I-5 near Roseburg area will be reduced from 65 to 60 mph. The truck speed limit there will stay at 55 mph.

Doug Bish, traffic services engineer for the state transportation department, said I-5 in Roseburg was a "blossoming crash problem" because of close-spaced intersections and high use of the freeway for travel within the city.

-- Elliot Njus

enjus@oregonian.com

503-294-5034

@enjus