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HDM-4 Key Parameters
1 Introduction
It is important for users to be aware of the general level of
sensitivity of the HDM-4 model to eachparameter so that appropriate
emphasis can be given to important parameters and less emphasis
tosecond or third order effects. The influences of individual
parameters differ according to theparticular parameter, the
particular result being considered, and the values assigned to
otherparameters in the particular analysis. The sensitivity of
results to variations in a parametertherefore varies somewhat under
different circumstances.
Sensitivity analyses were conducted to quantify the impact
elasticity of different HDM-4parameters. Impact elasticity is
defined as the ratio of the percentage change in a specific result
tothe percentage change of the input parameter, holding all other
parameters constant at a meanvalue. For example, if a 10 per cent
increase in traffic loading causes a 2.9 per cent increase
inroughness developed after 15 years, the impact elasticity term of
traffic loading for that roughnessresult is 0.29. If there were a
2.9 per cent decrease, the value would be 0.29.
On the basis of the analyses, four classes of model sensitivity
have been established as a function of the impact elasticity. The
higher the elasticity, the more sensitive the model predictions.
The mostsensitive HDM-4 parameters are given in the sections below.
Details of sensitivity classes aregiven in Volume 5 of HDM-4
Documentation series: A Guide to Calibration and Adaptation .
2 Road deterioration and works effects (RD and WE)
The RD and WE models predict the deterioration of the pavement
over time and under traffic.When road works are performed, the road
characteristics are changed and the quantities of materialapplied
are multiplied by their unit costs to establish the cost of works.
Table 1 shows the mostsensitive RD and WE parameters.
Table 1Most sensitive RDWE parameters
Parameter Impact elasticity Outcomes most impacted
Structural Number of Pavement
Pavement performance, resurfacing andsurface distress, economic
return
Traffic volume Economic returnDeflection (pavement) > 0.5
Pavement performance, resurfacing andsurface distress, economic
return
Road roughness Pavement performance, resurfacing andsurface
distress, economic return
Equivalent standard axle loads Pavement performance, resurfacing
andsurface distress, economic return
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3 Road user and environmental effects
These comprise vehicle operating costs (VOC), travel time,
safety, energy use and emissions.
When considering road user effects (RUE) there are two
situations that may be of interest, namelythe:
magnitude of the total RUE effects of operating conditions on
RUE
The former is important in situations such as when there are
different route lengths, while the latteris important when you are
comparing changes to road condition.
Table 2 shows the most sensitive road user and environmental
effects parameters.
Table 2
Most sensitive RUE parameters
Parameter Impact elasticity Outcomes most impacted
New vehicle price > 0.5 Total VOC and savings in VOCVehicle
Parts consumptionmodel exponent KP (vehicleage related)
Total VOC and savings in VOC
Road roughness 0.2 0.5 Total VOC and savings in VOCVehicle Parts
consumptionmodel coefficient a1
Total VOC and savings in VOC
4 Other important factors
The following factors also need to be examined carefully:
Traffic growth rates Relationship between the amount of surface
distress and the level of roughness Amount of generated traffic
Amount of exogenous benefits and costs Unit costs of vehicle
resources Unit costs of road works Value of time
