The impact of arc visibility on curve negotiation
A high percentage of serious accidents occur on sharp horizontal curves, especially on two-lane rural roads. A growing body of literature has examined driving behaviour on horizontal curves, with most research relating the effect of curve radius on driver’s speed and steering behaviour. There is an agreement that increasing degrees of road curvature result in less safe curve negotiation performance and consequently more accidents. Few studies, however, have further explored the effect of limited visibility on curve negotiation. This paper reports the results of a driving simulator study aimed at examining drivers’ behaviour on horizontal curves, in terms of speed and lateral position, in relation to varying levels of visibility of the curve’s arc. A two-lane rural road was designed and implemented in a desktop driving simulator and ten curve scenarios were examined with five different levels of visibility (20%, 40%, 60%, 80% and 100%), and of two curve radii (150m and 250m). Thirty drivers participated in the experiment; statistical analysis showed there to be a clear effect of radius on driver speed, as would be expected. However, when visibility decreased, reductions in driver speed were only found at the lowest level (20% preview). This speed reduction, however, was not sufficient enough for drivers to be able to negotiate the curve without detriment to their lateral positioning. Drivers tended to decelerate later and more sharply in the poor visibility curve and then have to compensate by moving towards the centre-line in order to flatten out the curve. It is concluded that whilst drivers can adapt sufficiently on curves that have moderate pre-view, when visibility deteriorates below a threshold (in this case 20% preview) drivers are unable (or willing) to reduce their speed appropriately and thus risk lane excursion.
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