The objective of this traffic safety investigation was to find critical road sections using Post- and Preaccident analysis approaches. The Post-accident approach analyzes the effect of road geometric characteristics on accident rate. The study was based on accident and road maintenance data in Western Sweden. A total of 2912 accidents from 2000 to 2005 on 1615 km median-separated roads was collected and combined with road characteristics (Speed, carriageway width, AADT, vertical and horizontal alignments, superelevation) for analysis (Post-accident approach).

The statistical analysis showed that road characteristics have great effect on accident ratio (AR defined as number of accidents per million vehicle kilometer

• AR and injury severity increase with increasing speed limit.
• A carriageway of 5,8m has the lowest AR, with a distinct tendency for AR to decrease with lane widths greater than 5.8m.
• AR decreases with increasing radii of curve for right and left-turn curves. Left-turn curves have higher AR than right-turn.
• Road sections with left-turn curve radii of less than 100m have highest AR;​ they are four times as high as those with curve radii greater than 500m and twice as high as right-turn curve radii less than 100m.
• The lowest AR were observed for superelevations of 3-4%. AR increases when superelevations increase or decrease from 3- 4%.
• AR on downgrades is higher than on upgrades.

In a Pre-accident approach the IST-Checklist method 2005 has been used. A tool based on human behavior that assesses a place or a road section’s inclination to trigger accidents. The purpose was to investigate applicability of IST on Swedish roads and then using it to find critical road sections. The results show that the method doesn’t function as expected for blind tests made on Swedish road sections which showed weak correlation between real accidents and IST results.

The investigation approach and results are useful input for designing future active safety systems such as ABS and ESP that are sensitive to the road characteristics.