How Psychological and Physiological Impacts Influence Traffic Interpretation

Author(s): Wayne Browne

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This paper reports the influence that psychological and physiological parameters have on the interpretation of traffic in an intersection where there may be conflicts between vehicles and pedestrians. A case study is presented where a bus collides with a pedestrian in a crosswalk, when both parties had a green signal, but the driver failed to yield. The bus driver was making a left hand turn at a four way intersection. This intersection is located at Main and King, streets located in Moncton NB. Main Street runs east and west, and King Street runs north and south. The accident occurred about 1 pm on January 20th, 2012.

The inference in this case is that there may be a causal explanation not being explored. The workload in this intersection is judge to be high with all of the common parameters such as high vehicular traffic volumes, obstructions, distractions and un-expectant influences as contributing factors in this accident. The conflict that occurred led to the inference that there may be an unknown contributing factor. A shadow and its intensity within the intersection were identified; this shadow changed in dimensions and location and light intensity throughout the day and from day to day. The light intensity was measured within the shadow at the time of day the accident occurred and was found to be 338 ft-candles. The light intensity outside the shadow in the intersection was found to be 9,660 ft-candles (a differential magnitude of a factor of about 30). This factor was tested throughout different light intensity periods and it was found when the light intensity differential varied between 25 and 50 fold there was an increased frequency of conflicts. This also was found to hold true when traffic enters a higher light intensity area from a lower light intensity during critical periods of the day, such as: just after sunrise when turning into the sun eastbound or just before sunset when turning into the sun, westbound. When you are exposed to a certain light intensity range for 20 minutes or more, as was the case for the bus driver, and you enter a lower or higher light intensity that is within the 25 to 50 fold differential it takes at least a half second for your eyes to adjust to the lower light level in the case of the bus driver or higher levels in other scenarios, however the vehicle enters such zones quickly. This period of reduced visibility affects your interpretation of the traffic and your response to that traffic due to this period of reduced vision. Since this period is short, not more than a half second, one is not conscious that it even occurred. This phenomenon can be mitigated by providing adequate lead time for the pedestrian to cross. This leading pedestrian interval (about 3 seconds) has been found to reduce conflicts with pedestrians.