Barrier separated road type design : accelerated degradation.

Barrier separated roads enable near-motorway safety levels to be achieved at a much lower cost than with actual conversions to motorway or dual carriageway standards. Transforming a high traffic volume road into a barrier separated road type reduces the number of fatalities by around 80 percent. A large percentage of the high traffic volume road network in Sweden has already been transformed into 2+1 roads. However, evidence has shown that barrier separated roads degrade quicker than conventional road designs. The extent of the problem was quantified by comparing various sets of road surface characteristic data. These comparisons revealed increases in rut development of up to 60 percent in the single lane sections of 2+1 type barrier separated roads. Vehicle position surveys were carried out and subsequent data analysis confirmed that vehicle confinement was a main cause of the problem. The analysis indicated that in the single lane section of a 2+1 type barrier separated road, vehicle lateral wander reduced by 24 percent for light vehicles and 19 percent for commercial traffic. These figures increased to 44 percent and 39 percent for a 1+1 type barrier separated road. Factors such as lane width, verge width, total width between road edge and central barrier, and close proximity of the central barrier all had some influence on vehicle position and the amount of lateral wander. In addition, the use of existing road surface materials may also contribute to the problem. Depending on traffic flow volumes, transforming a conventional road into a barrier separated road may result in increased specification standards. Use of an inferior existing surface could result in a 30 percent increase in surface wear. Accelerated degradation can be attributed to poor cross-sectional design. The amount of vehicular lateral wander is restricted and results in surface wear and loading being concentrated in narrow tracks along the road section. Design improvements are possible but will obviously require higher initial investment expenditure. (Author/publisher)