Research results have shown the following for rural, two-lane roads: 1. Increased width of carriageway/lanes, shoulders and roadway, up to certain limits, normally reduces accident rates. It must be observed, however, that it has hardly ever been possible to fully eliminate the effects of other safety affecting variables, and that many results therefore probably overestimate the safety effect of increased width. 2. Horizontal radii under 1000 m increase accident rates. The rate for a curve with a 400 m radius is about 50% higher than for one with a 1000 m radius. Steep grades also increase accidents. 4% grades show 20% higher rates than near horizontal sections. 3. Road sections with short sight distances show higher accident rates than sections with long sight distances. 4. Roadsides are very important to road safety. Front and back slopes should preferably be 1:4 or flatter. In rock cuttings the distance from the edge of the roadway to the rock wall should be at lest 6 to 9 m on high speed roads and 4 to 6 m on low speed roads. Clear zones, free from hazardous objects, should be sufficiently wide to avoid most collisions, that is, 7 to 11 m on high speed roads and 4.5 to 7 m on low speed roads. Hazardous objects should be eliminated or located outside the clear zone. 5. Junctions are specially dangerous spots in a road network. It is often advantageous to replace 4-way junctions by two staggered 3-way junctions. Islands in secondary road have a small positive safety effect. Left-turn lanes in primary road are favourable, at least if they are produced by road markings, and specially at 3-way junctions. Roundabouts and grade separation are very effective options from the safety point of view.
Abstract