has been noise regulation put in place to help control where and when the use of engine braking retarders are allowed. Operator health and safety A truck cab is a hazard control that protects the truck operator from hazardous airborne pollutants. As an enclosure, it is an example of an engineering control. Enclosed operator cabs have been used on agriculture, mining, and construction vehicles for several decades. Most modern-day enclosed cabs have heating, ventilation, and air conditioning (HVAC) systems for primarily maintaining a comfortable temperature and providing breathable air for their occupants. Various levels of filtration can be incorporated into the HVAC system to remove airborne pollutants such as dusts, diesel particulate matter (DPM), and other aerosols. Two key elements of an effective environmental enclosure are a good filtration system and an enclosure with good integrity (sealed isolation from the outside environment). It is recommended that a filtration system filter out at least 95% or greater of airborne respirable aerosols from the intake airflow, with an additional recirculation filtering component for the inside air. Good enclosure integrity is also needed to achieve positive pressure to prevent wind-driven aerosol penetration into the enclosure, as well as to minimize air leakage around the filtration system. Test methods and mathematical modeling of environmental enclosures are also beneficial for quantifying and optimizing filtration system designs, as well as maintaining optimum protection factor performance for enclosure occupants. Operations issues Taxes Commercial trucks in the US pay higher road use taxes on a state level than other road vehicles and are subject to extensive regulation. A few reasons commercial trucks pay higher road use taxes: they are bigger and heavier than most other vehicles, and cause more wear and tear per hour on roadways; and trucks and their drivers are on the road for more hours per day. Rules on use taxes differ among jurisdictions. Damage to pavement The life of a pavement is measured by the number of passes of a vehicle axle. It may be evaluated using the Load Equivalency Factor, which states that the damage by the pass of a vehicle axle is proportional to the 4th power of the weight, so a ten-ton axle consumes 10,000 times the life of the pavement as a one-ton axle. For that reason, loaded trucks cost the same as thousands of cars in pavement costs, and are subject to higher taxes and highway tolls. Commercial insurance