No. They are for any vehicle over a certain tonnage. Tandem trucks over a certain weight have to stop at weigh-stations. Examples would be gravel or grain trucks. It various from state to state or province to province. In some areas it begins at 5 tons. Edit - It is correct for Canada.

"Truck" Weigh Stations are for ALL commecial vehicles regardless of weight or size. Commercial busses are exempt. Weigh stations are not for private use by the motoring public, unless in the case of an emergency, or dictated otherwise by Law enforcment officers. Inside a "Truck" Weigh Station you will find Certified Professional Law Enforcement Officers who are fully equipped with side-mounted firearms. There might or might not be a vending machine available for food or drink. The building is heated and air conditioned and usually has a drinking fountain available for anyone who might be thursty. Public Pay telephones are almost always located outside on the back-side of the building. Anyone, including the motoring public, are allowed to enter a weigh station in an emergency situation. However, that does not mean that just because "John and Suzy Q Public" entered the ramp for the weigh station, that they need to cross the scale. The public can drive to the back of the building where there is a small automobile parking section. On rare occasions, when an officer finds it necessary at his/her own discretion, a public motorist or non-commercial farmer, or even an exempt commercial bus, can be forced to enter a weigh station if that officer believes that doing so is necessary for the safety of all other public motorists, or for his/her own discretion of reason to do so. When 18 wheelers enter and cross a weigh station scale, there is more taking place than "just weighing" the vehicle. The officer(s) inside the weigh station are also visually checking the 18-wheeler for any safety problems such as, a cracked or chafed air line or broken (you name it). They are also checking external sticker permits to ensure that the owner of the truck has paid the necessary highway use tax involved with commercial motor vehicle operation. Weigh stations operate through the tax funding. During harder economical times when a state can not afford to fund the operation of its weigh station(s). The weigh station will close and remain closed until funding affords the weigh station to operate again. This can be on a daily, weekly, monthly or yearly cycle.

One of the posters has provided an answer from his experiences in the US, I assume, because the answer is very different from the situation in Canada. Road transport falls within provincial jurisdictions in Canada, so each province has somewhat different regulations and practices. I am most familiar with the situation in Ontario and, to a lesser extent, Québec. Weigh stations are not open all of the time, so trucks are not required to pull into every station - only those that are open. Any commercial vehicle over a specified minimum weight, which can vary between provinces and with the seasons, is required to pull over and be weighed when the weigh station is open. The driver's log book may also be inspected. A safety inspection may be performed on any truck that does not appear to be in compliance with safety regulations. Roadside safety inspections are usually performed by officials from the Ministry of Transport (MoT), who have the right to inspect both the vehicle and the driver's log. The police may also stop trucks for safety inspections. (The police carry sidearms as part of their job, but MoT officials do not carry weapons to inspect vehicles in Canada.) A police car may be present at the weigh station, whether it is open or closed. The police station their vehicles at weigh stations and at other roadside locations when they are monitoring highway traffic. They may also provide support for Ministry personnel by stopping any commercial vehicle that does not pull into the weigh station for inspection when required. Most weigh stations also provide an emergency telephone, accessible at any time, for the use of road travellers during emergencies. These may also be found alongside some highways. Weigh stations seem to be more active in the spring, when the frost is coming out of the ground. Some secondary and tertiary roads reduce the maximum permissible axle load limits during spring, because the ground becomes wet and softens, increasing the damage from heavy axle loads.

No, they are for all commercial vehicles over a specified weight (usually 1 ton and over in some places) but it varies between the provinces and states, and is posted on a road sign at the side of the road, listing any exceptions, before the weigh station. Most also have a big road side sign with flashing lights to signify that the scale is open. If the lights are not flashing ... you bypass the scale without stopping.

not sure, never heard of them