Subject: Re: Speed differential vs. absolute

From: [email-masked] (nkibe)
Date: 17 Oct 1995 21:55:35 GMT

In article <[email-masked]>, [email-masked] (Mike Nakagawa) says:
>
>A while ago, I read that traffic with a large speed differential (something
>like a 15 mph spread) was more dangerous than when all the cars were
>driving a similar speed, even if the speed was fast.  For example, it would
>be worse to have traffic with speeds spread between 50 and 65 than to have
>everyone going at about 65.  (Assuming that traffic wasn't held up by two
>people in front going 65 with everyone tail-gating while waiting to pass.)
>
>Does anyone know if such a study existed, and if so, how I could read up
>on it?

A Dr. Cole (I think), GM Research Center, explored this topic in a
book on Highway Safety.  My local Borders carried it in the
Transportation Section, as might yours.

The book is technical/academic and quite dense.  However, I remember
that the treatment of safety on autobahns (where both absolute speeds
and differentials can be quite high) was a little weak, with pop
sociology explanations (Teutonic rigour, etc etc.) for the gap between
Dr. Cole's predictions and reality.

That safety decreases with high speed differentials is obvious.
I suspect that it also decreases with low speed differentials, 
particularly when combined with the collapse of lane discipline.

How so?  Because overtaking, yielding, exiting, merging require
space in the next lane, where another user might have chosen to
position himself and remain.  Low speed diffs mean that the time
for all of these becomes excessively long and one may simply run
out of road or block other traffic, hindering free flow.

Worse, if one treats only high speed diffs as dangerous, then law, 
enforcement and usage try to ensure perfect safety with zero speed
diffs, treating speed limits as limiting speeds (like that of light
in the physical universe, faster than which nothing can travel). 

But since cars are not physically linked and different drivers 
will never pick an identical "safe and reasonable" speed, some will
always be faster than others, irrespective of posted limits.

As a practical matter, this "irresistible force" of relatively faster
drivers would then clash with the "immovable mass" of law and usage,
creating chaos, if not a state of civil war, on the roads.

I think one can see this daily on US roads, worsened by its natural
corollary, the general collapse/non-enforcement of lane discipline.

Just personal views, of course.

Nitin Kibe.