Awareness of driving impairment: Lapses versus excursions out-of-lane during highway driving

Purpose: Next-day residual effects of hypnotic drugs may have a negative impact on the ability to drive a car. Lapses and excursions out-of-lane are two examples of loss of vehicle control that may result in car crashes. It can be hypothesized that drivers who experience loss of vehicle control try to maneuver the vehicle towards the safer road shoulder instead of the adjacent traffic lane (in which other traffic may be present). This may however not be true if drivers are unaware of driving impairment. The purpose of the current analysis was to determine the direction of lapses and excursions out-of-lane. Methods: Data from 152 driving tests from studies examining the residual effects of hypnotic drugs including zolpidem (10 and 20 mg), zaleplon (10 and 20 mg), ramelteon (8 mg) and zopiclone (7.5 mg) [1,2] was re-analyzed to identify lapses and excursions out of lane. The 100-km driving tests were performed on a public highway in actual traffic, and subjects were instructed to maintain a steady lateral position within the right traffic lane at a constant speed of 95 km/h. A lapse was defined as a deviation from the mean lateral position for more than 1 meter for at least 8 seconds [3]. The direction of lapses, i.e. over left (towards the adjacent traffic lane), over right (towards the road shoulder), or in both directions, was identified. Excursions out-of-lane are defined as inappropriate lane crossings, either onto the adjacent traffic lane (over left) or onto the road shoulder (over left). The frequency and direction of lapses and excursions out-of-lane was compared. As an indicator of driving impairment, the Standard Deviation of Lateral Position (SDLP) was also computed. Results: Both lapses and excursions out-of-lane were most frequent in drivers with clinically relevant driving impairment (i.e., SDLP increase relative to placebo greater than +2.4 cm). In total 308 excursions out-of-lane were recorded, of which 297 were over right (96.4%) and 8 over left (3.6%). In contrast, of the 300 lapses, 158 were over left (52.7%), 134 over right (44.7%) and 8 in both directions (2.6%). There were no significant differences between male and female drivers. Conclusions: Unawareness of driving impairment may have serious consequences for traffic safety. The current data showed that whereas excursions out-of-lane are predominantly made onto the (safer) road shoulder, lapses occur equally over left and over right. The latter may suggest that drivers are aware of loss of vehicle control during excursions out-of-lane, but not during lapses. Lapses may therefor put the driver at increased risk for having an accident. Objective measurement of arousal during driving (e.g. electroencephalograms) should verify if drivers are unaware of having lapses. Ideally, the occurrence of lapses is further related to accident data to determine whether lapses proceed and or coincidence with accidents.