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Objects
May Be Closer Than They Appear
(Researcher's comments appear in red)
This email is in response to Lab-Notes' question
of why people believe they are faster than
trains. I only have my own observations,
reasoning, and experience for my beliefs, so I readily admit that there
are probably
a number
of flaws.
First,
I believe our vision evolved for the hunting of game
smaller than us as well as for finding non-moving objects.
It is only in our most recent history (a few hundred
years) that single individuals have been able to hunt
large game. And even then, a successful hunter would
not be out hunting game that's charging directly at
him. The end result is that large objects look deceptively
slow because our brain is translating the large moving
object into what it expects - a smaller object. And
if it's
a small object, then it can't be moving too fast or
is farther away...
An interesting experiment would be to test people with
experience watching fast-moving objects (NASCAR professionals,
etc.) as well as people with experience watching large
objects (crane operators at docks, oil tanker pilots,
etc.), and compare those groups to a group of random
test subjects. Will those with experience have a better
perception of speed than the standard person?
I like Cummins thinking here. We hadn't thought of
looking at folks like NASCAR drivers, but my expectation
is that they would do a better job of judging speed,
just as postal workers are terrific at judging weight.
We're hoping to do a variant of what he proposes, to
see how normal observers judge speed of large trains
and also of small rail-capable pick-up trucks on the
same tracks.
Second, it's psychology. Most people (especially in
this day and age) don't believe anything bad can happen
to them - more so if they've gotten away with it previously.
Or, if someone sees someone else get away with an action,
they are prompted to do it for themselves no matter
how stupid it seems (the incorrect lemmings-over-a-cliff
scenario). I see this quite often - not only at railroad
crossings but also at stop signs and crosswalks.
Others
have written similarly. I'm no expert on matters of motivation
or risk taking but I wouldn't be surprised to find out
that some collisions are caused in this way.
And then on other, related matters...
While the size of buses may be a contributing factor
for the number of collisions they're in, I believe
Professor Cohn has neglected one very important
factor about buses - they are not required to have a
center
high-mounted stop lamp (CHMSL, also called "the
third brake light"). Beginning in 1984, the
CHMSL was required lighting equipment for all new cars in the US whose purpose
was to put brake light directly in a following
driver's line of sight and thus help reduce the
number of
rear-end collisions. However, this piece of safety
equipment
was not required for trucks, buses, or any other
vehicle greater than 2032mm (80in) wide. There
are a number
of buses that do have a CHMSL system, but since
there are no Federal regulatory guidelines about
where
to place them on the bus, the lamps are often placed
too
high to be of use for a following driver in near
proximity to the back end of a bus.
I
concur with the writer. It is now established that CHMSL's
have lowered rear-end collision rates (by about 4% if memory
serves) for passenger vehicles. However, we don't have
a good idea as to where to place them on buses. CHMSL placement
on passenger vehicles works (for many vehicles) because
the following driver is trying to look through the car
ahead and so a CHMSL in the rear window is likely to be
near where he/she is looking.
I've seen Professor Cohn's 'Bus Bar' at a previous Cal
Day and thought it was nothing more than a glorified
CHMSL (and possibly a safety hazard for having
TOO many lights flashing which will distract drivers - NHTSA discourages
the use of flashing signals except in the case of emergency
warning or of turn signaling).
Unfortunately, only a grad student with marginal experience with it was
available to explain the project so I had to leave my
many questions about it unanswered.
I would welcome Professor Cohn's response to my views as well an opportunity
to read the papers supporting his theories.
I am a lighting professional with 10 years experience as a photometric
engineer. Also, I happen to live next to the Emeryville Amtrak with a view
overlooking
the train station as well as an auto/train crossing. If I can be of any
service, I would enjoy helping any research study.
Douglas
Cummins
Calcoast - ITL
BS ME/NE '93
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Both fighter pilots and parachutists know the problem of
judging distances. When fighter planes join-up in formation,
the closure rate can be over 100 mph and with no background,
only sky, there is no other reference for judging distance
other than the plane being joined. This results in the plane
being joined appearing to be stationary and not growing in
size. Then, suddenly the plane being joined seems to "blossom" or
grow very rapidly as the stereopsis becomes effective. The
reason for this non-linear rate of change is that the human
eye does not measure the angular change of the object that
subtends its view very effectively. It is particularly ineffective
when there are no other visual clues to supplement the rate
of change, such as telephone poles, buildings, etc. Therefore,
the fighter pilots have developed an overshoot procedure
so that collisions do not occur, although with experience
and using other information, such as the known speed of the
aircraft to be joined and their own airspeed, they can make
better judgments as to when the "blossom" is going
to occur. Aircraft equipped with radar can utilize this equipment
to give distance and closure rate as an aid adjusting for
a smooth join-up.
This phenomenon occurs at road crossing as well. Just about
everyone has had a car, which was required to wait at a STOP
sign until you pass, sit at the STOP sign for a considerable
time then suddenly pull out in front of you. Again, your
car appeared to the waiting car to not be growing any larger
and therefore still at a safe enough distance for it to enter
the road.
Parachutists have the same problem. If they look directly
down at the ground, they do not get any sensation of their
downward speed until very near the ground, and then they
get a "ground rush," which can result in injury
if they instinctively raise their legs and land on their
buttocks. To counter this limitation, they have learned to
keep their eyes on the horizon and wait until their feet
actually touch the ground before they execute their landing
maneuver.
As to whether large objects pose a greater problem in overtake
recognition: This is an interesting postulation, which could
be explained by the eye having even more trouble in measuring
the rate of change with large subtended angles then with
smaller ones. Of course, knowing that the object being observed
is large, as a bus, has the effect of receiving misinformation.
One expects the bus to be big and until the stereopsis effect
appears, one feels comfortable with the apparently safe separation.
With regard to a warning system using the lights on the train:
The problem is not speed or distance, directly. It is time,
the time for the train to arrive at the crossing. A very
effective solution would be to install radar on the train
with a reflector at each crossing. The radar information
could be coupled with a computer and the train's lights.
When the train was more than two minutes from the crossing,
the lights would be their normal bright white light needed
for seeing ahead. At two minutes to the crossing the lights
could change to amber, at one minute change to red and at
30 seconds become a flashing red. By using the radar and
a computer with the changing lights, the crossing traffic
would always know the critical relationship, time, with the
oncoming train regardless of its speed and distance.
The radar, computer and lights would not have to be a very
complicated system to provide this valuable information,
and not prohibitively expensive. Further, there is new technology
on the way. See Business Week, October 20, 2003, page 137,
Super-Radar, Done Dirt Cheap.
Grove A. Rathburn (MS Mining 1959)
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