South Dakota's emphasis on vehicle weight enforcement has focused attention on the fact that such equipment often exceeds legal load limits if loaded with commodities, and sometimes even when unloaded. For convenience and economy, equipment users prefer driving such equipment over state and local roads (typically surfaced with gravel or asphalt pavement) to off-road work locations. Because of regular and seasonal load limits, they must often transport equipment and commodities separately at extra labor and expense. Equipment users have questioned whether the load limits are appropriate, in light of the fact that many of the off-road vehicles are designed to minimize soil compaction in off-road use. In lay terms, the question is "If this equipment is designed to reduce soil compaction, and can run over small plants without damaging them, how can it possibly damage roads as much as ordinary truck tires with the same weight per axle?"
Unfortunately, the effects of such loads and tire configurations on pavements are not well known. Most models of pavement damage are derived from road tests using conventional truck tires, and cannot predict pavement response under other tire configurations, pressures, or footprints. Without knowledge of the effects of off-road equipment on typical state and local pavements, it is impossible to assess the financial impacts of its use, and to determine whether present regulations are too strict, too loose, or appropriate.
Research is needed to evaluate the physical effects of such loads on pavements, to assess economic impacts, and to provide a basis for possible regulatory changes.
Findings: Field tests showed that application of heavy loads supported by off-road tires caused significantly higher levels of vertical deflection and pressure in the base and subgrade of flexible and granular pavements. On flexible pavements, significantly higher levels of strain at the bottom of the hot mix asphalt layer were also observed. Using a computer model validated by the field testing, the research showed that such loads would be expected to cause very significant increases in pavement damage on a wide range of flexible and granular surfaces that exist on South Dakota's state and local highway networks. On the basis of field testing, model verification, and modeling of structural and climatic conditions that exist in South Dakota, researchers concluded that no relaxation of current vehicle weight restrictions was warranted.