Here’s a question that has puzzled me for a while. You see, I do a lot of cross-country driving (for various reasons) and I am always seeing those “weigh stations” on the side of the interstate. However, probably 2/3 of the ones I see are closed. What gives? Did they construct too many and now budget cuts forced them to close? Why are there so many in the first place? And are trucks supposed to stop at each one they pass, or just one per state, or one per day, or something?
The US government taxes load carrying vehicles above a certain tonnage for the additional damage that they cause to the roadways. The weigh stations are how they monitor the relative usage by these vehicles. They also serve as checkpoints for determining whether the truck is being operated safely and if the driver is getting enough rest.
I think to save money weigh stations are manned only sporadically and randomly. This would also be in the interest of keeping things moving. Stopping trucks every 50 miles (or whatever) would increase the costs associated with shipping things.
However, a trucker never knows when he might run into one of these things so it is in their interest to make certain their trucks meet whatever laws are relevant where they are driving.
Finally, a question related to my job!
First of all, weigh stations are used for two purposes; weighing of course, but they’re mostly used now for safety inspections.
The weigh stations are built everywhere so that the local enforcement authority (usu. the department of transportation or highways, or the ministry of transportation in a Canadian province) can run blitzes. Essentially, what this means is that they’ll not only switch from station to station doing weight checks and safety inspections, but at times they’ll open up damn near all of them for a few days as part of a big safety blitz.
The stations were never meant to ALL be open all the time, though there are 24/7 stations here and there in key places. Having them available and ready for use allows you to run blitzes or adjust to new traffic patterns.
The people who work in the stations have other duties too, including facility audits on carriers (e.g. actually going to a trucking company’s facility and ensuring they meet the law - a “carrier” is the industry’s word for a trucking company) and doing roadside inspections. So it’s not like they’re sitting at home doing nothing if the station is closed.
Most weigh stations signal trucks in with lights; if you look as you go by them they have a green/red light out front that tells trucks to pull in. They’ll just randomly select trucks sometimes, or pull some in and inspect bad looking ones, or whatever the current project calls for.
Trucks always have to stop IF they’re signalled in. However, if they’ve been recently inspected, they probably won’t be inspected again (though they’ll likely be weighed.) When a truck passes an inspection it’s given a CVSA (Commercial Vehicle Safety Alliance) sticker that indicated it’s been recently inspected. Inspection personnel in the US, Canada, and Mexico all get the same training and perform the same inspection, so they can trust the stickers to some extent. Weight laws differ from jurisdiction to jurisdiction, however, so that’s a different story; if they’re pulled in they’ll almost always get weighed.
If you go to the CVSA web site at http://www.cvsa.org there’s actually a picture on their front page of a guy holding up a CVSA sticker. Of course, a real inspector would be wearing coveralls, or he’d be sort of greasy.
I live in Northern New England. Up until a year or two ago, the weigh stations were always closed. Then a Canadian truck driver tried take out a few bridges in downtown Boston. (Whenever we have a major truck accident, it seems to involve a driver from Quebec whose improperly licensed, driving an overloaded truck, or just loaded). My state got a federal grant to modernize the stations. Now they’re open for safety inspections practically every other day - but only on the southbound side of the highway. 
To this day, I’ve never seen the northbound station open.