Adequacy of barrier and median separation on rural roads

Abstract

This study reanalyses the data from research by Doecke and Woolley (2010) that examined road departures with regard to clear zones and roadside barriers, in the new context of wide medians and median barriers (i.e. departures to the right side of the direction of travel). This study includes data from 62 rural crash investigations conducted from 1998 to 2010 in rural areas to which an ambulance was called. Many of the vehicles in the sample of in-depth crash investigations collided with fixed objects within 15 metres of the roadway yet still about 13% traversed 15 metres of lateral width from the roadway and had an occupant who needed to be transported to hospital. Of those 10 cases where no fixed object was struck, only five came to rest within 15 metres of the roadway. Computer simulations were performed based on five of the cases, three ‘drift off’ type run off road to the right crashes and two single yaw run off road to the right crashes. Each case was simulated using two different driver scenarios. The simulations revealed that wide medians may cater well for vehicles which drift off the road but those that lose control before departing the road will be likely to cross the median. Furthermore, the vehicles that lost control were found to still be travelling at between 55 and 70 km/h after crossing a 15 metre wide median. The computer simulations were also used to examine the appropriateness of median barrier offsets. The barrier was placed at a range of different lateral offsets from the road to examine the optimal positioning of roadside barriers on both wide and narrow medians. In general, smaller barrier offsets produced less severe impacts but all barrier offsets tested would be acceptable. It was concluded that a 15 metre median would cater for drift off crashes but not loss of control crashes. Therefore, to create a true safe system median barriers must be considered. It is also desirable to have a median that is not as wide as currently recommended when a median barrier is installed. Barriers on narrow medians will safely prevent departures to the right. The exact barrier offset (and therefore median width) on a narrow median had little effect on safety.