Beginning the the early 20th century, the 1920's saw several major advancements. There was improvement in rural roads which was significant for the time. The diesel engine, which are 25-40% more efficient than gas engines were also a major breakthrough. We also saw the standardization of truck and trailer sizes along with fifth wheel coupling systems. Additionally power assisted brakes and steering developed. By 1933, all states had some form of varying truck weight regulation.
All cars must pass some sort of emission check, such as a smog check to ensure safety. Similarly, trucks are subject to noise emission requirement, which is emanating from the U.S. Noise Control Act. This was intended to protect the public from noise health side effects. The loud noise is due to the way trucks contribute disproportionately to roadway noise. This is primarily due to the elevated stacks and intense tire and aerodynamic noise characteristics.
In the United States, commercial truck classification is fixed by each vehicle's gross vehicle weight rating (GVWR). There are 8 commercial truck classes, ranging between 1 and 8. Trucks are also classified in a more broad way by the DOT's Federal Highway Administration (FHWA). The FHWA groups them together, determining classes 1-3 as light duty, 4-6 as medium duty, and 7-8 as heavy duty. The United States Environmental Protection Agency has its own separate system of emission classifications for commercial trucks. Similarly, the United States Census Bureau had assigned classifications of its own in its now-discontinued Vehicle Inventory and Use Survey (TIUS, formerly known as the Truck Inventory and Use Survey).
In 1938, the now-eliminated Interstate Commerce Commission (ICC) enforced the first Hours of Service (HOS) rules. Drivers became limited to 12 hours of work within a 15-hour period. At this time, work included loading, unloading, driving, handling freight, preparing reports, preparing vehicles for service, or performing any other duty in relation to the transportation of passengers or property.
The ICC intended for the 3-hour difference between 12 hours of work and 15 hours on-duty to be used for meals and rest breaks. This meant that the weekly max was limited to 60 hours over 7 days (non-daily drivers), or 70 hours over 8 days (daily drivers). With these rules in place, it allowed 12 hours of work within a 15-hour period, 9 hours of rest, with 3 hours for breaks within a 24-hour day.
The Federal Motor Carrier Safety Administration (FMCSA) issues Hours of Service regulations. At the same time, they govern the working hours of anyone operating a commercial motor vehicle (CMV) in the United States. Such regulations apply to truck drivers, commercial and city bus drivers, and school bus drivers who operate CMVs. With these rules in place, the number of daily and weekly hours spent driving and working is limited. The FMCSA regulates the minimum amount of time drivers must spend resting between driving shifts. In regards to intrastate commerce, the respective state's regulations apply.
Heavy trucks. A cement mixer is an example of Class 8 heavy trucks. Heavy trucks are the largest on-road trucks, Class 8. These include vocational applications such as heavy dump trucks, concrete pump trucks, and refuse hauling, as well as ubiquitous long-haul 6×4 and 4x2 tractor units. Road damage and wear increase very rapidly with the axle weight. The axle weight is the truck weight divided by the number of axles, but the actual axle weight depends on the position of the load over the axles. The number of steering axles and the suspension type also influence the amount of the road wear. In many countries with good roads, a six-axle truck may have a maximum weight over 50 tons (49 long tons; 55 short tons).