Road Roughness Test

For many years, the ride experienced by the occupant of a vehicle has been used as a criterion of pavement roughness. A person travelling on a vehicle at a selected speed subjectively evaluates the riding experience. The vibration of the vehicle induced by road roughness affects driver performance. It has been reported by human factors researchers that continuous exposure to vibration may induce fatigue, which may in turn be a factor that contributes to accidents. It is mentioned that vehicle suspension also plays a role in the effect of roughness on safety. Frequency has a major effect on automobile and trucks that can cause safety problems. When roughness occurs at a repeated amplitude and frequency, it can have two effects. First, if the frequency is in phase with the vehicle, a rough road can seem to be smooth. Second, if the roughness is out of phase with the natural frequency, the roughness can be magnified. This phenomenon causes a problem when repeated upward forces continually reinforce already existing upward forces of the vehicle. When this occurs, the vehicle starts to move up and down with the roadway in ever increasing movements until the car or truck tyres actually leave the roadway and become airborne. The roughness of the pavement causes change in the normal forces that act at the tyre-pavement interface and affects the lateral forces needed to control a vehicle, thereby influencing the steering capabilities of the vehicle. On relatively smooth pavements, the variation in normal force is very small. However, on rough pavements, these variations can be very large and reduce significantly the lateral force available to control the vehicle. In situations where high lateral forces are necessary or where a vehicle requires a particular lateral friction level to make a manoeuvre, this loss of force could lead to loss of control of the vehicle. Roughness can cause significant loss of braking force or slip resistance on a vehicle. In addition to this, problems can arise due to variation of roughness in each of the wheel tracks of a lane. These differences of roughness in each wheel path cause differences in the braking and sliding forces that act on the vehicle. This will cause the exposure of vehicle to different levels of friction on each side, which can result in a significant safety problem.

The Vehicle Operating Cost (VOC) components consist of fuel consumption, oil consumption, tyre
wear, maintenance and repair, and depreciation. Road surface roughness is one of the factors determining Vehicle Operating Cost (VOC). Roughness affects the amount of fuel consumed in vehicles. While increasing roughness increases fuel consumption at any given speed and reduces travel speed. In Highway Development and Management Model, HDM-4, effect of International Roughness Index (IRI) on Vehicle Operating Cost (VOC) was studied, by a set of notional 1km road lengths with IRI ranging from 2 to 2.5 in increments of IRI 0.5.

It is important to analyse the relationship between roughness and speed, as well as speed and fuel to determine the relationship between fuel consumption and roughness at variable speed. Various studies examining the roughness-speed relation have established a negative correlation between the two as roughness serves to decrease ride comfort and drivers reduce their speed accordingly. HDM models shows that at low speed, increasing speed serves to decrease distance-related fuel consumption because a greater distance is covered during each unit of time-related fuel consumption. Repair and maintenance cost also gets affected by road roughness.

Vehicle’s ride quality varies with travel speed, the dynamic characteristics of the vehicle, and the spectral characteristics of the roughness of the road. Assessment of ride quality and road roughness by measuring the response of a vehicle is dependent on travelling speed and dynamic characteristics of the vehicle. To obtain repeatable and reproducible measurements of ride quality, the measuring speed required to be controlled. Depending on the type of instrument and application, the measurement speed ranges from 20 to 80 km/h.