In article om,
"Dave Liquorice" writes:

On motorways the average traffic levels appear to dictate if it's lit or
not.

A few years ago, I asked this question of a chap from TRL who
I meet very occasionally. Traffic levels do figure, but it's
not that simple. It depends on the expected accident levels
when the motorway was built or at last major refit. But it
also depends how much money is available and if it runs out
before the job is completed (as happened on some dark sections
of the M25 which were supposed to be lit). Streetlamps do make
a significant contribution to accident reduction particularly
at intersections, but going in and out of lit areas itself does
seem to raise accident rates, so you can't just put them at
the odd accident blackspot.

I suspect the economics may be changing too. 20 years ago, they
were in effect almost free to run in terms of power, as they
used the spare overnight capacity of our nuclear plant which
was otherwise wasted. This isn't true anymore and they do cost
a lot to run nowadays. As I often do motorway runs at 2am and
find the roads empty with thousands of lamps lighting my way
into the distance, I do wonder how long that may continue.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]