The effect of road roughness on traffic speed and road safety

Road Safety is one of the most important issues for traffic authorities, as they attempt to reduce the frequency and severity of road crashes. This dissertation investigates the effect of Road Roughness on traffic speed and road safety in Southern Queensland, Australia. This research is important as it investigates the pavement variable of roughness, and considers its contribution to vehicle speed and crash rates. Using data collected by the Queensland Department of Transport and Main Roads, models were formed to depict the relationships between roughness, speed, crash rate and other road parameters. The model focused on a sample of roads in the 'Downs South West' region. The models collectively indicated a strong relationship between higher crash rates and increased pavement roughness. Road segments with a crash history have a higher average roughness than non-crash segments. Crash rates involving light vehicles were more affected by increasing roughness than crashes involved heavy freight vehicles. When considering the five crash severity types, crashing resulting in hospitalisations and property damage had the strongest increase in crashes over a small increase in roughness. Regarding driver speed, there is 100% driver compliance on segments with roughness over 120 counts/km NRM, with the 85th percentile speed ranging from 5-15km/hr below the posted speed. The models presented similar conclusions to Australian and International research, but produce slightly different results from the two similar published investigations. Crash rates showed a steadily increasing linear relationship with increasing roughness and are slightly higher than Swedish results, however are well below the critical crash rate as specified in the MUTCD. This suggests that Queensland's road safety procedures are being implemented effectively. These findings can be utilised by traffic authorities managing rural roads to create a safer road environment. Recommendations include ensuring regular road surface maintenance to provide low roughness (an IRI of 1.9m/km). Providing incentives to contractors for delivering a smooth pavement over the design life will ensure better pavement and construction quality. Prioritisation for maintenance of roads with lengths of roughness over 120 counts/km NRM may be suitable, with temporary speed reductions applied until works are completed. Prioritising maintenance on routes with lower volumes of heavy vehicles may also be suitable. Each of these recommendations can be implemented to ultimately improve the safety on the road network, through efficient funding prioritisation and understanding the effects of pavement roughness