One of the most fundamental ways to classify commercial vehicles is by considering where a commercial vehicle performs its primary job or vocation. Whether a vehicle operates only inside a city with its frequent slow speed, stop and start conditions, off- road, or at steady high speeds encountered during on-highway operation, or in a combination of several operating environ- ments, it has a major influence on vehicle design, operation, and maintenance. The choice of its engine, how well it needs to maneuver, its requirements for traction on various road or off-road surfaces, and service and inspection intervals are just a few variables affected by a vehicle's operating environment. Each of these environments can also have further subcategories. For example, truck delivery, or even bus operations, can be classified as regional or long haul. Regional operations return more than 90% of vehicles back to a terminal in a single day. Long-haul operations dispatch vehicles on trips lasting days- and even weeks-before they return to a dispatch, loading, or maintenance facility.
As already mentioned, road surface conditions also shape the classification of vehicles. That is, vehicles can be classified according to their suitability for travel on various road surfaces or grades. As shown in FIGURE 1, road grade is expressed as a percentage and refers to the steepness of a hill. For example, a 15% grade means the road drops or increases 15' (4.6 m) for every 100' (30.5 m) traveled.
Similarly, heavy-duty commercial vehicles can be classified according to how frequently they start and stop, or even by their speed of travel. When classifying commercial vehicles by operating conditions, five broad categories are used: Turn- pike or interstate; on-highway; on-off-highway or mountainous highway; off-highway; and urban.
Turnpike or Interstate
Major highways are constructed differently than secondary highways. The road materials, vehicle speeds, amount of stop- and-go driving, and slope of the road crown are all factors that determine a vehicle's suitability for turnpike or interstate appli- cations. Those factors also influence the design, equipment, and accessories an interstate vehicle uses. Vehicles classified for operation on interstates, highways, or turnpikes are designed to run on limited-access, well-maintained, multi-lane highways made of excellent concrete or asphalt with maximum adverse grades not in excess of 3%.
On-Highway
Vehicles classified for use in on-highway operating conditions operate exclusively on well-maintained major highways of excellent concrete or asphalt construction. Roads are typically level to rolling with occasional maximum grades to 8%. These vehicles must operate within legal weight and dimensional lim- itations and be capable to handle the steeper grades that may be encountered.
On-/Off-Highway or Mountainous Highway
Vehicles classified for on-/off-highway or mountainous high- way operating conditions should expect to spend 20% of their total operating time on secondary roads. These roads are normally made from good concrete or asphalt. Intermit tent grades of up to 12% may be encountered. The remain- ing time is traveled on off-highway roadways that are based on well-maintained crushed rock or similar material. Road grades in this class are more severe than on-highway opera tion. Operations are subject to legal weight and dimensional limitations.
Off-Highway
In this classification, vehicles spend more than 20% of their travel time on gravel roads or roads with a maintained crushed rock surface. The maximum grade can be as much as 12%, and grades of 8% are frequently encountered. This oper ation is not generally subject to legal weight or dimensional limitations.
Urban
Vehicles in this classification face operating conditions that are primarily within cities and suburban areas. That means these vehicles are subject to frequent stops and starts. Oper ation occurs on concrete, asphalt, and maintained gravel road surfaces. Because of the urban environment, vehicles in this class must have greater levels of maneuverability than is required of other classes. Vehicles in this class often use engines with a lower power output and transmissions with more steps.