Friction measurement methods and the correlation between road - VTI

Figure 4.14 Multiplier for locked-wheel stopping distance (referencetemperature is 25 o F = -4 °C, 0 °F = -18 °C).Accidents reported to the police, traffic measurements, and observations ofroad conditions were used for assessing the accident rate at different conditions.Table 4.1 shows accident rates, from Wallman et al. [53], the assessments werebased on data from Nilsson [34].Table 4.1 Accident rates at different conditions (accidents per million axle-pairkilometres).Bare road,all yearBare road,summerOctober - AprilBare roadIce/snowNorthernSwedenCentralSwedenSouthernSwedenDaylight 0.37 0.41 0.28 1.07Darkness 0.83 1.38 0.60 1.43Daylight 0.46 0.51 0.39 2.67Darkness 1.01 1.69 0.76 3.30Daylight 0.55 0.59 0.49 5.29Darkness 1.06 1.48 0.91 7.93The accident rates in Table 4.1 are averages over the year or over the winterseason, so it is not possible to associate them with any friction numbers. However,the geographical differences are clearly visualised.Data on winter road maintenance measures, accidents reported to the police,and vehicle mileage was used by Sävenhed [50] to estimate the accident ratebefore and after maintenance measures. Figure 4.15 shows a comparison betweende-icing and anti-icing (precautionary) actions with salt. De-icing actions areundertaken when the friction has decreased, while precautionary actions shouldpreserve good friction. The increased risk for the de-icing actions is probablyentirely related to impaired friction. Precautionary actions imply much lessvariations in the accident rate.VTI meddelande 911A 39