Driving with hemianopia, II: lane position and steering in a driving simulator.

The loss of half the field of vision in homonymous hemianopia (HH) could affect several driving skills, including detection of traffic-relevant objects, steering and lane position, and eye and head movement. In a previous paper,1 we reported the effect of HH on detection of pedestrians while driving; this second paper addresses the effect of HH on steering and lane position. The ability to steer a vehicle so that it remains on a steady course within the travel lane is an important aspect of safe driving. Steering behavior that results in incursions into adjacent travel lanes endangers both the driver and other road users. Problems with unstable steering and lane-keeping have been reported as the most common reasons for failure of on-road tests by HH drivers.2,3 Those findings were based on the most commonly applied scoring methods used in on-road studies: examiner's comments2 and observer's ratings of steering and lane position.3 However, those methods do not measure actual lane position. One of the advantages of using a driving simulator is that steering and lane position can be quantified for all drivers in exactly the same conditions. In an early simulator study, Szylk et al.4 reported that HH drivers made more lane boundary crossings and had greater variability in lane position than did normally sighted drivers. However, only 5 minutes of driving was evaluated, and the small sample (n = 6) included drivers with visual neglect, making it difficult to isolate the effects of the visual field loss from the effects of visual neglect. Furthermore, the lane position measures were apparently averaged across the entire drive without differentiating the various types of roadway segments (straight, curve, turn). A normally sighted driver's lane position varies with roadway curvature: driving close to the center of the lane on straight segments5 and moving to a leftward position in left curves and to a rightward position in right curves.6 The same was reported for drivers with peripheral visual field loss7 (not due to HH), but not for drivers with central visual field loss, who showed little variation in lane position with changing road curvature.7 For drivers with HH, the side of the hemifield loss may also affect lane position, and there may be interactions with the road segment type. Therefore, we evaluated steering and lateral lane position of drivers with HH and normal vision (NV) in a simulator for specific road segments (straights, right and left curves, and right and left turns) when driving on the right side of the road (as in the USA.). Our primary hypothesis was that drivers with HH would take a lane position that increased the safety margin on the blind side. Starting with this hypothesis, we then made specific predictions about the lateral lane position of drivers with HH for each road segment type. On straight road segments we predicted that they would provide a safety margin by taking a lane position away from the blind side: drivers with RHH would take a leftward lane position, whereas drivers with LHH would take a rightward lane position (relative to NV drivers). The predictions were more complex for curves and turns. For curves, our predictions depended on whether the curve opened into the blind or seeing hemifield. Drivers with NV usually take a rightward path on right curves. We predicted that drivers with RHH, who would be driving into their blind right field, would maintain a safety margin on the blind right side by staying to the left of drivers with NV. Conversely, we predicted that drivers with LHH (curve opening into seeing field) would have a rightward lane position similar to that of drivers with NV. On left curves, drivers with NV usually take a leftward path. In this situation we predicted that drivers with RHH would take a leftward path similar to that of drivers with NV (curve opening into their seeing hemifield), whereas drivers with LHH would maintain a safety margin on the blind left side by staying to the right of drivers with NV. We made different predictions for turns. Unlike curves, a turn is usually initiated only after visual scanning to detect potential conflicts with other traffic and to ensure that the path through the intersection is free of obstacles. The main task is then to steer so that the vehicle exits the intersection into the correct lane and avoids danger from the median curb or oncoming traffic at the end of the turn. When turning right, the danger of oncoming traffic at the end of the turn starts in the right field and gradually switches to the left field. Therefore, on right turns we predicted that drivers with RHH would take a more rightward (tighter) path than drivers with NV, bringing the risk zone of the oncoming traffic (the left lane boundary or median) more quickly from the blind right field into the left seeing field (compared with drivers with NV who maintain the risk zone in their seeing field throughout the maneuver). For LHH, we predicted that the path would be similar to that of drivers with NV, or even slightly more leftward, keeping the risk zone in the seeing right field for as long as possible. On left turns, the oncoming traffic at the end of the turn starts in the left field and remains there throughout the turn. Therefore, we predicted that drivers with RHH (risk zone in the seeing hemifield) would take a path similar to that of drivers with NV, whereas drivers with LHH (risk zone in the blind hemifield) would take a more rightward (wider) path to ensure a safety margin on the blind left side, especially at the end of the turn arc where they must avoid entering the oncoming traffic lane (or the median curb).