Subject: Roadway Lighting - MH vs HPS

From: [email-masked] (M. Clay Belcher)
Date: 23 May 1995 14:28:58 -0400

I just wanted to check in again on the great roadway light source debate.  I
would like to hear from those involved in roadway lighting around the U.S. as
to what source you currently prefer and why - Metal Halide or High Pressure
Sodium?  What is actually being installed in your area?  What trend do you
see for the future?

Here in Kansas it's quite a mixed bag.  I am doing an investigation for the
Kansas Department of Transportation to survey what they currently have,
assess how it's performing, and make recommendations for the future.  My task
is limited to the Interstate system.  On the Interstates we have primarily
Metal Halide, high mast lighting, but there's a fair amount of HPS too, as
well as the occasional left-over Mercury Vapor. There is currently no
well-defined policy regarding new installations or future direction (hence
the reason for the study).

At this point we will probably be recommending that all future roadway
lighting be Metal Halide, and that existing HPS be retrofit to MH as ballasts
fail.  This is based on what we see as two emerging areas of concensus.  1)
"Scotopically enriched" light sources (i.e. more power in the blue/green part
of the spectrum) produce better visibility at low light levels.  Thus on an
equal lumen basis (since the lumen is defined by V lambda - photopic vision)
sources like Metal Halide will provide more visibility than High Pressure
Sodium.  2) People simply prefer whiter light as we found in another study
which I hope to present to IESNA in August (regarding a telephone survey of
streetlighting users in Kansas City).

Now of course the first argument could be overturned if the economics were so
greatly in favor of HPS that we could provide equal visibility (taking into
consideration the scotopic effect) with HPS at lower cost than with MH.  In
other words, (taking a purely ficticious example) if it took twice as many
lumens to produce a given level of visibility with HPS as with MH, but it
only cost 40% as much per lumen to install, generate, and maintain the HPS
system, then HPS would still be more economical.  This does not seem to be
the case, but even if it were, we would still have the second argument to
deal with.  How much is people's preference worth, anyway?  (A topic I've
advanced some thoughts on before, as some of you know).

Anyway, I just wanted to see what directions others might be headed on
this issue.

Thanks,

Clay Belcher, PhD, P.E.                 voice:  (913) 864-3434
Associate Professor                     fax:    (913) 864-5099
Architectural Engineering               e-mail: [email-masked]
University of Kansas