Blind spots in vehicle mirrors
Jearl Walker www.flyingcircusofphysics.com
December 2013 The purpose of a vehicle’s side-view mirror is to allow you to see a trailing vehicle in an adjacent lane. However, many arrangements of a flat mirror leave a blind spot, which is a region in which the trailing vehicle is too close to be visible in the mirror. The danger is that you may switch lanes without being aware of a nearby vehicle. The vehicle might be slowly passing you (it could disappear from view for several seconds) or traveling at your speed (it is then continuously hidden from view). I have some photos to illustrate the situation but if you want something more startling (especially if you are a parent), skip to the video link farther down.
Here is a sketch of the blind spots on two sides of my car (I drive on the left side) with two flat side-view mirrors.
I can initially see an overtaking car with the rearview mirror, then eventually with a side-view mirror, and then finally with my peripheral vision as the car pulls alongside me, either in the adjacent lane or the next lane over. The danger lies in the interval when the car is no longer visible in my rearview mirror and before it is visible in my side-view mirror. If it is moving rapidly while I change lanes in its direction, there can be a collision before either the other driver or realize my error.
I arranged two stationary cars in my driveway so that when I photographed through the rear-view mirror and through the right-hand side-view mirror of the lead car, the trailing car was not visible. Here is what I see through the rear-view mirror.
And here is what I see through the right side-view mirror.
The trailing car is not seen through either mirror. Here is the arrangement as seen in front of the cars.
While sitting in the leading car, I could, of course, turn my head sharply to look back along the adjacent region. However, taking my eyes off the road at high speed is itself dangerous. Besides, the headrests and side panels framing the rear window obscure my view, requiring a longer look for me to understand if a car is there.
A safer but simple solution is to mount the side-view mirror on the front of the car hood , as was done with some sports cars. Although this gives a much broader view of the adjacent lanes, the mirror is so far from the driver that the image is small for any reasonable size of mirror. Another solution is commonly used on trucks: A convex mirror is added to the side-view mirror to give a broad view of the adjacent lanes. Again the image is small but now the mirror is close enough to the driver to be useful.
I use a third solution. I quickly lean forward while I glance at the side-view mirror. I can then see any car in the blind spot.
The situation on my driver’s side of the car is about the same. Again I've arranged for there to be a trailing car but this time on my left side. Here is the view through the side-view mirror on the left side.
And here is the scene from in front of the cars.
Here again a car can disappear into a blind spot, but I can see it by leaning forward.
Here is a video that more dramatically demonstrates a blind spot, this time to a truck driver and in spite of the fact that the side-view mirror (on the left side on this United Kingdom truck) has a convex portion.
Dots · through ··· indicate level of difficulty
Journal reference style: author, journal, volume, pages (date)
·· Quadling, D., "What the eye doesn't see, ..." Mathematical Gazette, 71, 198-201 (1987)
·· Clifford, F., "More on drivers' blind spots," Mathematical Gazette, 73, 120 (1989)
· Luoma, J., M. Sivak, and M. J. Flannagan, “Effects of driver-side mirror type on lane-change accidents,” Ergonomics, 38, 1973-1978 (1995
··· Hicks, R. A., and R. K. Perline, “Blind-spot problem for motor vehicles,” Applied Optics, 44, No. 19, 3893-3897 (1 July 2005)
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