Research Papers

How differences in ambient light intensity in or near an intersection will affect the interpretation of traffic ahead specificity vulnerable road users.

Version 1
Date added June 18, 2019
Downloaded 0 times/fois
Category 2019 CARSP XXIX Calgary
Tags Research and Evaluation, Session 2C
Author/Auteur Browne
Stream/Volet Research and Evaluation
Slidedeck Presentation Only:



Background/Context: On January 20th 2012 at the intersection of King and Main streets in Moncton NB, a fatal collision between a city bus and pedestrian occurred at approximately 1300h that afternoon.
Considering that this accident involved a professional driver, a bus driver, with many years of experience infers that there might be unidentified circumstances that contributed to this tragic accident.
This intersection during this time of year and day has a shadow that spread half-way across this four land intersection. This also occurs two other times of the year at this location. This is the result from sunlight being blocked by adjacent buildings south of the intersection. On that particular day the light intensity for south bound traffic from North and down King Street onto the intersection at Main Street is found to be brighter than a portion of the intersection. At the intersection stop light on King Street, the ambient light intensity was measured to be ~9600 ft-candles and the ambient light intensity in the shadow within the southern half of the intersection on Main Street was measured to be ~360 ft-candles. The question that arose was, did that shadow contribute to the cause of this accident?

Aims/Objectives: 1. To test the light intensities before entering and within the shadow;
2. To test what light intensity differential difference that causes a loss of vision of the traffic ahead;
3. To determine how much time loss occurred during this loss of vision;
4. To determine how much recovery distance is required to react to traffic ahead.
5. To determine how often this occurs.
6. To determine what affect this will have on a driver's interpretation of traffic ahead.

Methods/Targets: Under controlled conditions, a driver's reaction times was measured under a change of light intensities differentials. This was performed by using a controlled switching device that switch either a red flag or a green flag, at random, to be on when the test vehicle entered the shadow from bright sun light. The distance for the driver to reach a complete stop was measured, when a red flag was displayed. A control was establish, for a bench mark, to determine the driver's reaction time, when there was no shadow present. These test were repeated 10 times for each scenario.

Results/Activities: It was determined that the driver took an additional, on average, (2.13m) or 7 feet more or a second longer to stop, when travelling ~ (16km/h) or 10 miles per hour, when the red flag was displayed, when compared to the control stopping distance. This occurred when the light intensity differential between bright sunlight and when compared to the shadow has reached a factor of 1/28 in light intensity reduction.

Discussion/Deliverables: When traffic crosses a zone in an intersection where the light intensity varies; the light intensity varies instantaneously, however, a driver's eyesight adjustment to this change does not. It takes a minimum of a second for a normal eye to adjust and longer for others public and when severe conditions occur. Due to this physiological conditions, reaction time to perceive, to interpret and to respond can vary.

It was documented that these conditions occur in a significant number intersections that have an obstructions to sunlight causing shadow within the intersection. The light intensity differential occur during different periods of the day and year in these intersections experience light intensities that differ with a differential factor of 28 or 1/28.

Conclusions: It was rationalized that the cause for a vehicle to take an additional 7ft to stop when the light intensity was suddenly reduced by a factor of 1/28 was the result for the eye to adjust to a reduction of this ambient light intensity. This delay to perceive will affect the time to respond to traffic ahead.
Any delay in perceiving traffic, will contribute to traffic misinterpretation. This affects all road users, especially vulnerable ones, if inadequate separation between road users is not provided.
To compensate for this delay a second or more of travel distance separation between road users is required."