In 2016, 30% of new cars sold in Sweden were fitted with All-Wheel-Drive (AWD). However, there is limited research on the real-life safety effects of AWD. The objectives of the present study were to:​ (i) calculate whether AWD reduces the risk of involvement in injury crashes among cars fitted with Electronic Stability Control (ESC);​ (ii) evaluate if AWD has any influence on impact severity and speed;​ (iii) investigate the winter tire fitment among AWD cars involved in injury crashes.

Swedish police records for the period 2003-2016 were used (STRADA). Only cars with ESC were included. AWD cars (n=5220) were identified and matched with the 2-Wheel-Drive (2WD) version of the same car models (n=21827) or other similar 2WD cars (n=8799).

Different methods were used for each objective. (i) To calculate the risk of being involved in an injury crash, an induced exposure approach was used, where AWD-sensitive to AWD-non-sensitive crashes and road conditions were matched in relation to cars with AWD and 2WD. (ii) To estimate the impact severity and speed, the paired comparison method for 2-car crashes was used. The relative injury risk for each group of cars was calculated by comparing the injury outcome for that group with the injury outcome for the vehicles they collided with. The relative difference between the impact severity for AWD and 2WD cars was translated into a difference of impact speed using the Power Model. (iii) To investigate the winter tire fitment, the present data were merged with a previous study also based on STRADA. In that study, additional information on winter tires fitment was collected from a sample of drivers using a questionnaire;​ 290 cases were included in the present study population. The results for roads covered with ice or snow showed that injury crashes increased by 19-31% with AWD. Similar results were found for head-on and single-vehicle crashes. No significant difference was found between Permanent and Automatic AWD. On icy or snowy roads, AWD cars had a 13-15% higher impact severity than 2WD cars, which corresponded to an 8-10% increase of impact speed for AWD cars. On dry or wet roads, no differences were found between AWD and 2WD. Although based on a limited material, the survey indicated that AWD and 2WD cars had similar distributions of winter tires.

The results suggested that AWD may lead drivers to underestimate the level of available friction on icy or snowy roads and therefore to drive at faster speeds than they would do with a 2WD car. Therefore it is recommended that AWD should not be advertised as a safety feature. The necessity of fitting AWD on a wide range of car models should be carefully reconsidered. AWD technologies should be further developed so that slippery road conditions are not disguised by the increased traction provided by AWD.