In July of 2007 I posted an article on this (Gorski Consulting) website entitled “Roof Pillars Can Be A Visibility Obstruction”. The point was to illustrate that a driver’s view could be obstructed at an intersection by a roof pillar and this could lead to a collision. This conclusion was based on some testing that was performed in a 1999 Chevrolet Lumina passenger car. It demonstrated that at highway speed, a zone of about 5 car lengths along the crossroad could be invisible to the driver because of the blockage posed by a roof pillar.
While this result is important the visibility obstruction that exists in other vehicles such as large trucks and buses is even greater. This fact is rarely discussed. Collisions occur with other vehicles from this factor. But more importantly collisions occur with more vulnerable victims such as pedestrians and cyclists because no one has made this point known.
The following example will provide a detailed focus on the problem.
School Bus Versus Pedestrian Collision
To illustrate the circumstances by which a pedestrian could be struck by a school bus the following photo of a collision site is shown followed by examples of the visibility available in a school bus and then how an impact might occur as a result of the limited visibility.
The photo below shows an intersection of two roadways where the driver of a school bus attempted to make a left turn along the general line shown in the measurement of the school bus travel path of about 25 metres.
Measurements of a similar school bus were taken to document the extent to which the bus driver’s vision would be blocked by the left exterior mirror and the left roof pillar. The photo below shows an exterior view of a similar school bus. The left roof pillar and exterior mirror can be seen.
As shown from the interior of the bus below the width of the area of obstruction by the mirror and roof pillar was about 23 centimetres.
Another measurement was taken to determine the bus driver’s eyes with respect to the obstructing mirror and roof pillar.
Measurements indicate that a school bus driver’s eyes would be located about 2.4 metres to the rear of its front bumper, about 2.4 metres above the ground, and about 0.65 metres to the right of the left interior wall of a bus. These values will vary slightly dependent on the adjustment of the driver’s seat which changes for each driver’s preference.
Measurements like these allowed for the creation of a scale diagram, shown below, which determined the area of obstructed sight produced by the mirror and roof pillar. With the school bus starting its motion at the bottom of this image we examined the view obstruction after the bus advanced 7.5 metres and 22 metres. Measurements of the site indicated that the travel paths of the pedestrian and school bus were initially about 7.5 metres apart. As both units advanced forward there was a continual zone of about 3.43 metres within which the pedestrian was not visible to the bus driver.
However, as the bus began its turn into an angle of about 30 degrees the bus driver’s eyes began to get closer to the pedestrian and, combined with the change in the bus angle, this caused the zone of obstruction to diminish to about 1.1 metres. This would be a critical moment. If the bus driver was looking in the direction where the pedestrian emerged from the obstruction he might be able to apply his brakes or steer away to avoid striking the pedestrian. But such success cannot be guaranteed. The driver’s attention could to directed toward other areas of the exterior environment at this precise instance. Even through he might revert his attention quickly back to where the pedestrian is walking a delayed reaction could result in a failure to avoid a collision.
A further appreciation of the collision conditions can be obtained by taking a replica school bus to the actual collision site to explore the experiments that were conducted in the scale diagram analysis.
The photos below show a reenactment of the collision scenario where we see progressive motions of the school bus taken from the driver’s seating position as the school bus approaches its left turn.
As the school bus approaches the intersection the driver would steer the bus into the left turn lane. Unfortunately in this re-enactment this motion was not possible and the motion being shown is with the bus remaining in the through lane. Never-the-less the visibility scenario is similar to the actual event. The orange circle in the image shows the approximate location of the pedestrian before she began to walk southbound across the road.
In the next images we see the bus progressing forward slowly as the driver is looking into the opposing lanes of traffic to determine when a reasonable gap may develop that would allow the bus driver to complete his left turn. This attention conflict is common in driving environments where a driver may need to be aware of more than one possible conflict in different areas of the exterior environment. In these images we also include the orange circle identifying the position of the pedestrian who is intending to cross the road.
As shown in the photo below, as the school bus advances the view of the position of the pedestrian becomes obscured by the exterior mirror and the roof pillar of the bus. If the pedestrian began to move forward onto the road at the time shown in the photo below the bus driver would not be able to see that this motion of the pedestrian had begun.
In the photo below it can be seen that, given the specific conditions of a slow forward motion of the school bus, the bus driver would not be able to see the motion of the pedestrian because that pedestrian’s motion would be blocked by the exterior mirror, roof pillar, or both.
So long as the school bus continued forward at a similar speed as the pedestrian then the pedestrian would remain hidden behind the mirror/pillar of the bus. At some point the bus would begin its left turn and this would expose the pedestrian into the bus driver’s line of sight. If the bus driver was not looking in the area where the pedestrian became visible he might not stop in time and the pedestrian could be struck.
School buses are not the only large vehicles that contain zones where it is difficult for the driver to detect the presence of a pedestrian or cyclist. The hood edge of many large vehicles is so high that it could hide the presence of a small child that is near the vehicle’s front bumper. In other instances the drivers of heavy trucks making right turns cannot see the progress of pedestrians or cyclists along the right side of the truck and/or its trailer.
Not only is it important for drivers of heavy vehicles to know where these zones are, but it is equally important that pedestrians and cyclists be aware of where these zones exist and in what circumstances they may not be seen by the driver of a heavy vehicle.