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S.E. Hassan, T. Jiang, D. Sauerburger, G.D. Barnett, R.W. Massof; The Discriminability of Vehicular Gap Times: Comparing Visual and Auditory Information to Only Auditory Information . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3684.
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Crossing the street is a complicated, high–risk activity of daily living. In making an appropriate decision of when it is safe to cross an unsignalized street, pedestrians must judge gaps in vehicular traffic that is of sufficient duration to allow them to reach the other side of the street or splitter island. Little is known however how well pedestrians can judge differences in gap duration based on using both visual and auditory information compared to judgments made with only auditory information. The aim of this study was therefore to compare the discriminability of different gap durations based on using both visual and auditory information compared to using auditory information alone.
Using a 5 point rating scale, safety ratings for vehicular gaps of different durations were measured along an unsignalized, two–lane street of one–way traffic. Safety ratings were collected from 2 normally sighted subjects for 7 different gap times under two sensory test conditions: (i) visual and auditory information; and (ii) auditory information only. For each tested gap duration, a Receiver Operating Characteristic (ROC) curve was fitted to the rating data with the hearing only rating data on the ordinate and the vision plus hearing rating data on the abscissa. The discriminability (d’) of the street–crossing safety decision variable for the two sensory test conditions was calculated as the area under the ROC curve for each of the 7 gap times.
For both subjects, d’ was zero or negative for gap durations of 4 seconds or less and was positive and increased monotonically with gap duration for durations longer than 4 seconds.
Positive values of d’ indicate that the distribution of the street–crossing safety decision variable is lower for the hearing alone condition than it is for the vision plus hearing condition, and vice–versa for negative values of d’. If the decision variable represents an estimated margin of safety, i.e., the difference between the subject’s estimated crossing time and estimated gap time, then these results suggest that relative to vision plus hearing, subjects under–estimate the margin of safety for gap times longer than 4 seconds and over–estimate the margin of safety for gap times of 4 seconds or less when relying on auditory information alone.
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