Re: University Physics

[Larry Woolf <larry.woolf@GAT.COM>]

Regarding the 160 ft. headlight illumination:
My reference is an email and chart that I received from the research and
evaluation division of the National Highway Transportation and Safety
Administration (NHSTA).

Regarding reaction times:
There is an interesting discussion of reaction times in the book "Traffic
Safety and the Driver."  If you are anticipating an event, reaction times
can be as short as 0.15 s.  While driving, your reaction time is divided
into a perception reaction time ("I need to brake") and a movement reaction
time (movement of your foot).  For drivers focusing on the car ahead of
them, an average reaction time is 1.6 s.  For drivers encountering an
unexpected obstacle around a blind curve, an average reaction time is closer
to 2.5 s.  NHTSA also considers the 1.5 s reaction time to be appropriate.

It seems to me that mechanics problems that have some bearing on driving
safety are in fact quite relevant. Students are starting to drive at the
same time they are starting to learn physics. With this particular problem,
they can actually understand why most country roads have 35 mph speed
limits.

I have written an education module on this topic
(<http://www.sci-ed-ga.org/modules/driving/index.html>) and will be
presenting at the California Science Teachers Association Meeting in
October, if anyone on the list would like to attend and receive a copy.

Larry Woolf;General Atomics;6995 Flanders Dr.;MS 78-107;San Diego CA
92121-2975; Ph:858-526-8575;FAX:858-526-8568; www.ga.com; www.sci-ed-ga.org

-----Original Message-----
From:  Hugh Haskell
Sent: Tuesday, September 24, 2002 1:37 PM
Subject: Re: University Physics


At 12:47 -0700 9/24/02, Larry Woolf wrote:
> Headlights illuminate the road up to 160 ft in front of you.  If you are on
> a road with stop signs, what is the fastest speed you can drive and still
> stop safely at night?
> A typical reaction time for braking is 1.5 s (See Traffic Safety and the
> Driver by Leonard Evans) and a typical braking acceleration for a car is 17
> ft/s2.

This is certainly the same problem that Tina first posed, just
dressed up in PC clothing. But I suspect that there are a couple of
features of it that may make it somewhat less relevant than it
appears as first glance. First, at least in my benighted area, all of
the stop signs are done with ScotchLite, or some generic equivalent
(I'm sure MMM's patent has expired by now), which makes them visible
from substantially more than the "illuminated distance" (and many
cars now use halogen headlamps which have a range of more than the
"standard" headlamps). The 160 feet range would be more
characteristic of non-reflective objects, like animals (who aren't
looking at you), or pedestrians without proper reflective vests.

And second, the reaction time seems rather long to me. I have done
reaction time tests with my students over the years and they seem
able to achieve values of well less than .5 sec.