<P> Truckers often refer to weigh stations as "chicken coops ." </P> <P> Many states now use electronic bypass systems (or AVI - Automatic Vehicle Identification) to alleviate some of the truck traffic through the weigh station . Some of the best known are PrePass, NORPASS and Drivewyze . The system may consist of equipment at the weigh station itself, as well as a truck mounted transponder or smartphone, usually placed on the inside of the windshield or on the dashboard . These are similar to transponders used for toll collection . Each transponder is directly registered to a specific truck, and contains a unique identification . The registration process propagates information such as carrier name, unit number, and elected gross weight to weigh stations . In addition, the system keeps a basic safety and compliance record for each vehicle . As a truck approaches a weigh station (approximately one mile before), an electronic "reader" on a boom over the freeway reads the information from the truck transponder . It also looks at the safety and compliance record on the database . A display shows the results to the weigh master, including the speed of the vehicle . The weigh master may have the system automatically determine if a truck needs to stop or may override the system . Approximately one - half to one full mile after passing under the "reader", the truck will pass under another boom which has an electronic unit to send the transponder a signal . If the safety information is acceptable the truck may receive a green light and can continue without entering the weigh station at all . There are weight detecting devices in the roadway itself . A driver may get a red light . On these occasions, the truck must pull into the weigh station for the normal weigh - in procedure . The most common reason a truck is "redlighted" is a weight problem, or a random check . Each time a truck is randomly pulled in, it is noted in the system whether the driver was compliant or not during the check . This affects how often a truck (or different trucks from the same company) are pulled in . For example, a company which is very compliant with the law will probably only have 5% of its trucks "redlighted ." </P> <P> Weigh Stations (aka "Scales") are usually on the right - hand side of the travelled highway, but median scales are appearing (as of 2005) on divided highways, often combined with "weigh - in - motion" technology . </P> <P> A median scale is placed between the opposing lanes of traffic, necessitating heavy vehicles exiting from the left lane (rather than the right) and re-entering traffic from the left, potentially at a lower speed than the normal "free - flow" traffic would expect in the left (often thought of as "fast") lane . </P>

A truck weighing station has a single scale