Some traffic lights have the sign posted, "left turn on red."  This
means it is legal for drivers to turn left during a red light.  How
many traffic lights exist in the United States where drivers can
legally make a "left turn on red"?

ZGeller --

These left turns on a red light, are usually FROM a one-way street
INTO a one-way street.  And they're often in dense sections of city,
where the "left turn on red" is meant to avoid gridlock.

Of course you understand that any other condition would either put the
L turn across on-coming traffic (when coming FROM a two-way street
onto a one-way -- or from a one-way onto a two-way).

Example #5 is diagrammed in this California DMV page:
California Driver Handbook
"Rules of the Road"
http://www.dmv.ca.gov/pubs/hdbk/pgs25thru29.htm

Here's an interesting Geocities site which summarizes the law by each
state.  Justin JIH, says that it's legal in 42 states and Puerto Rico,
though the page is not dated, so it's difficult to tell the timeliness
of the information:
http://www.geocities.com/jusjih/trafficlightsignals.html


Google search strategy: 
left turn on one way street
left turn on red light

Best regards,

Omnivorous-GA

---

Clarification of Answer by omnivorous-ga on 05 Nov 2004 11:53 PST

ZGeller --

I realized afterwards that I'd missed the "number" of these
intersections.  I'm not sure that I know how to calculate the number
of traffic-lighted intersections.  I just tried (and there was a
recent question here that went unanswered too.)

There's probably somebody very smart about mapping and GEOS software
who could find intersections of two one-way streets.  Then screen out
the 8 states that don't allow the turns. . .

Don't hesitate to use the Clarification Request process in the future
before rating an Answer.  Sometimes we can dig a little deeper, even
though I can't get a handle on this one.

Best regards,

Omnivorous-GA

This answer didn't answer my question, but in fact clarified it in a
way showing that it would very difficult to answer.  It was an
educational answer, but didn't attempt to give a numerical answer. 
That would be pretty hard - but isn't that right up Google's alley?