Evaluation of the use of 80 km/h advisory speed signs on unsealed roads in South Australia

A number of 80 km/h advisory speed signs were placed on unsealed roads in the Yankalilla area in South Australia in November 2014. As part of the evaluation of this trial, two weeks of vehicle speed measurement data was collected at 12 sites on these roads in June 2014 and again in January 2015. While the legal speed limit for these roads was 100 km/h, typical vehicle speeds at the measurement sites were well below this limit with typical mean speeds being around 60 km/h and only around 10 per cent of vehicles exceeding 80 km/h and less than 1% exceeding 100 km/h. Based on this, it is clear that a 100km/h travel speed is not suitable for these roads and that an 80 km/h maximum advisory speed sign is not out of place. A reduction in vehicle speeds (using various measures) was found after the advisory speed signs were installed. However, the interpretation of this reduction is complicated by the seven month gap between the two surveys. There were significant changes in the road surface conditions between the surveys on some roads and in fact the roads with improved surfaces also tended to exhibit higher speeds. When roads with obviously changed surface conditions were taken out, the reduction in vehicle speeds was considerably greater. However, it is not possible to disentangle the seasonal effects from the effect of the advisory speed signs using the current data. Another survey conducted at the same locations in June 2015 would be useful as it would remove the seasonal effect from the comparison.