Empirical measurement of the Enright phenomenon, a distortion of perceived driving speed

Abstract

Purpose: In 1970, Enright observed a distortion of perceived
driving speed, induced by monocular application of a neutral
density (ND) filter. If a driver looks out of the right side of a
vehicle with a filter over the right eye, the driver perceives a
reduction of the vehicle’s apparent velocity, while applying a ND filter over the left eye increases the vehicle’s apparent velocity.
The purpose of the current study was to provide the first empirical
measurements of the Enright phenomenon.
Methods: Ten experienced drivers were tested and drove an automatic
sedan on a closed road circuit. Filters (0.9 ND) were placed
over the left, right or both eyes during a driving run, in addition
to a control condition with no filters in place. Subjects were asked
to look out of the right side of the car and adjust their driving
speed to either 40 km/h or 60 km/h.
Results: Without a filter or with both eyes filtered subjects showed
good estimation of speed when asked to travel at 60 km/h but
travelled a mean of 12 to 14 km/h faster than the requested
40 km/h. Subjects travelled faster than these baselines by a mean
of 7 to 9 km/h (p < 0.001) with the filter over their right eye, and
3 to 5 km/h slower with the filter over their left eye (p < 0.05).
Conclusions: The Enright phenomenon causes significant and
measurable distortions of perceived driving speed under realworld
driving conditions.