We’ve all seen the open and closed signs for highway weigh stations. Have you ever wondered what, exactly, happens at those stations? And do truckers always stop, or do they sometimes avoid them?

Usually, a weigh station is just that: a place where big trucks are weighed to make sure they are not overloaded. An overloaded truck is a major hazard on the road. Among other things, they are harder to steer and take greater distances to slow down or stop. In some states, the bigger the weight violation, the bigger the ticket: In Massachusetts, for example, it’s $40 for every 1,000 pounds above the permitted weight, but if the truck is more than 10,000 pounds overweight, it’s $80 per 1,000 pounds over.

In addition to checking a truck’s weight, state police often set up truck inspections at weigh stations. They will check trucks for all sorts of safety violations: Do the truck’s brakes work properly? Do its brake lights work? Are the tires in good shape? Is its load strapped down properly? Is the vehicle leaking anything? Is the driver’s paperwork in order? This includes registrations, weight permits, his driving record, medical information and logbook to make sure he hasn’t been driving too many hours. They will also check the company’s safety record.

But if a weigh station is open, and a trucker knows he’s in violation, won’t he just avoid that weigh station or wait until it closes? Probably. And because of their CBs, truckers know when the stations are open or closed. So, many states are trying out portable inspection stations. They plop down portable scales at random points along a road or highway, or near intersections with high incidents of truck accidents. There they can perform all the inspections they would at a regular weigh station — and also give out tickets for moving violations.

In Massachusetts, these portable inspection stations have been credited with helping reduce by 20 percent the number of fatal trucking accidents in the state. In fact, for accidents involving commercial vehicles, Massachusetts now boasts lowest fatality rate in the United States.

So the next time you drive by a closed weigh station, remember that there may be a portable inspection station somewhere else doing an even better job of finding trucks with problems.