On Thu, 7 Aug 2008 10:24:41 +0100, FCS wrote
(in article
<9ecf28d4-f3cc-42a0-be52-1ef964dec95e@34g2000hsf.googlegroups.com>):

As nightjar has already said, they *should* be set for vehicle activation -
which means that on quiet roads they will spend a lot of time with both ends
of red. When a car approaches, that end will immediately turn green because
the system doesn't need to wait for the other end to turn red etc. For simple
two way lights, there are two timers that can be adjusted - one sets the
maximum length of a green signal in each direction, the other sets the
duration of all red (ie both ends are red while traffic clears the works
before the other flow begins).

During a discussion with the managing agent on one of my local trunk roads
following a complaint, the agent sent me a PDF of the manual they work to for
TTLs. It's quite interesting and makes it clear that almost all the TTLs I
come across on our busy trunk road are not legal - simply because the
clueless f***wits that set them up have no idea what the twiddly bits are for
or how to set them.

One of my pet hates, and the source of my complaint mentioned above, is
numpties who set up TTLs on a busy road but don't adjust the timers.

The all red time needs to be adjusted for the length of the works - too short
and you get one end turning green before traffic from the other end has
passed, too long and you waste capacity by having no traffic flowing at all.

Then the green time needs adjusting as well for the flow. The shorter the
green time, the more often the flow changes ends, and the more time you spend
on all red. If the traffic flow is high, you need to increase the green time
to reduce the proportion of all red in the cycle. I've seen them set so that
only half a dozen vehicles get through on a cycle and there's a tail back for
miles.

If the flow is very high, then the manual calls for manual control.

Just to throw some numbers into the game (I know they are short**, it's just
to make the numbers easy) ...
Set the all red to 10s and the green to 10s. Assuming traffic from both ends,
then the overall cycle time is 40s (one green at 10s, plus all red for 10s,
plus the other green for 10s, and another all red for 10s) and each end gets
green for 10s - ie a max flow rate of just 25%% compared with not having the
lights.
Increase the green phase each way to 40s and now the total cycle time is
100s, and each end gets green for 40s - the max flow rate has gone up to 40%%
Now increase the all red to 20s while leaving the green at 40s. The total
cycle time is 120s and each end gets 40s of green - flow rate is down to 33%%

On the occasion I mentioned above (it's not the only time I've complained
about misused TTLs), some contractor had set up four way lights (crossroads)
with short greens each way and long red times. It was a damn good job we'd
allowed an extra hour to get to the concert we were going to as it took a
full 1 1/2 hours to get through these lights. I hope the contractor got a
right bollocking from the managing agent - apparently they were only licenced
the lights on condition of manual control, but I guess they simply knocked
off on Friday evening and set them to auto.

Unfortunately, I don't think the highways agencies are prepared to do
anything about it. IMO, when the lights are misused (ie not set in accordance
with standing instructions), then the lights should be considered an
obstruction and the people responsible prosecuted - a few prosecutions and I
think we'd soon see things done better.

** Actually, in Barrow there are numerous new lights on the crazy one-way
system some idiots have imposed. One set really is green for something like
10s - just long enough for 3 cars, or 4, 5, or 6 counting the ones who ignore
the amber or even red !