The Scottish Road Network Landslides Study made a number of recommendations for landslide management and mitigation. Amongst these was that ‘wig-wag’ signs could be suitable at sites with specific conditions, particularly those sites at which debris flow events occur on a regular basis. Such signs incorporate a standard rockfall/landslide red warning triangle, flashing lights, and a sub-plate that that warns of ‘Higher risk when lights flash’. A trial of such signs commenced on the A83 in 2011 and was centred on the section of the route that includes the Rest and be Thankful. As part of this trial it was intended that a detailed technical and perceptual evaluation of the signing arrangements be undertaken. An initial technical evaluation, after two years of operation, and a perceptual evaluation were undertaken. This report extends and updates the technical evaluation to four years of operation and examines the efficacy of the operation of the wig-wag signing arrangements at this location. The previous perceptual evaluation explored the perceptions and attitudes of road users to these specific signs. It is broadly concluded that the trial has had a satisfactory outcome; 15 of the 17 debris flow events (almost 90%) that occurred during the period of the trial were within a period when the lights were activated, or most likely activated in the two cases when the time/day of occurrence is not known with precision. The two events that did not occur during such a period were of a somewhat different character, but must nonetheless be seen as ‘false negatives’. The period during which the lights were activated and were not associated with a debris flow event corresponds to between 12% and 19% of the days of the year; this is significant but is considered to be broadly acceptable in the light of the rainfall triggers that are currently available to determine the timings of switch-on and switch-off. The results of the previously reported driver perception element of the work indicate that, in general, desired behaviours are promoted by the use of the signs. The A83 Rest and be Thankful locality is known for the frequency with which debris flow events occur, much more than any other part of the trunk road network in Scotland. It is thus well-suited to the use of this type of temporal warnings. The potential application of wig-wag signs to other parts of the network is limited and any proposals should be the subject of detailed location-specific assessment. The evidence presented in this report supports the continued use of the wig-wags and the continued promulgation of messages about desired behaviours to the non-local driver target audience. (Author/publisher)
Samenvatting