Illustration of the FHWA 13-Bin Vehicle Classification Scheme, Texas Department of Transportation
FHWA developed a standardized vehicle classification system in the mid-1980s. This system was the result of compromises designed to meet the needs of many traffic data users. Pavement designers were an important segment of those users but by no means the only intended audience. Another segment of key users comprised the safety community, which was (and still is) highly interested in the amount of travel occurring in multi-unit vehicles (that is, power units of various types pulling trailers of various configurations).
In addition to these needs was the requirement that the electronic equipment and sensors available at the time (mostly simple road tubes) be able to differentiate passing vehicles into the desired classifications. Available sensors were capable of measuring the presence of vehicles, detecting axles, and determining the distance between consecutive axles on the basis of the speed of each vehicle as it passed over the sensors.