I have been driving between Medford and Ashland throughout the summer and I have noticed that not all trucks seem to stop at the weigh stations on Interstate 5 between the Ashland exits. Why is it that some trucks have to stop and others don't? Furthermore, what about the times when the weigh station is closed?

I have been driving between Medford and Ashland throughout the summer and I have noticed that not all trucks seem to stop at the weigh stations on Interstate 5 between the Ashland exits. Why is it that some trucks have to stop and others don't? Furthermore, what about the times when the weigh station is closed?

— Ben D., via e-mail

Ben, we've often wondered the same thing. Secretly, we've wondered whether the weigh stations were actually disguised Starbucks for truckers.

There actually are two reasons why you see trucks zooming by. Either they're breaking the law, or they're equipped with a nifty device called a "Green Light" transponder.

Oregon has a weight-mile tax, so heavy commercial vehicles have to stop and be weighed for tax purposes. Besides the Ashland weigh station, there are 86 other fixed weigh stations in Oregon.

A commercial truck with a "regular load" weighs between 10,000 and 80,000 pounds. If it weighs more or doesn't meet the width and height requirements, the driver must purchase an over-dimension permit.

"The cost of the permit and fees varies depending upon the size and weight of the vehicle and the time frame of the permit," said Sally Ridenour, an ODOT spokeswoman. "Permits start at $8."

Some trucks can avoid the time and hassle of stopping at a weigh station by having a Green Light transponder, a little electronic device that's mounted on a truck's windshield or dashboard. Only 21 stations in Oregon have this weigh-in-motion capability, and most are along major highways.

"There are scales on the highway with an electric eye that reads the transponder and weighs the vehicle in motion," said Ridenour.

"If it turns red, there is an issue and they need to come in and be weighed," she said.

This device cannot be purchased. To qualify for the Green Light Program, a truck must receive a "satisfactory" safety rating, have a permanent Oregon motor-carrier registration, visit Oregon automated weigh stations at least once per month and have at least three axles with a minimum declared gross vehicle weight of 34,001 pounds.

Many of the weigh stations have cameras installed that capture the license plate number of truckers who bypass an open weigh station and don't have a transponder. The fine for this misdemeanor violation is up to the local court, but it could range from $427 to more than $2,000 depending on the case, Ridenour said.

A weigh stations' open hours are set at random to ensure compliance. ODOT opens more weigh stations on days with heavy traffic. A driver needs to stop only if the station is open. If he or she has a Green Light transponder, they're good to go.