A guest post by Andrew Morse, our senior traffic engineering consultant
There, I’ve said it and I feel much better. Well not really because every day I am
astounded by the number of drivers who think it’s perfectly acceptable to enter
an intersection (typically on amber) knowing full well that they can’t clear
the intersection and are going to block the intersecting traffic. And what do they gain? Ending up in front of
a few cars making the left turn from the side road! Well, they would be turning
if you hadn’t stopped in their way!
It’s easy to be flippant about queuing through
intersections and treat it as nothing more than silly drivers getting annoyed
with each other, but there is a more serious side to this issue, which I am
convinced, is getting worse by the day.
In Sydney and most major cities, traffic signals are controlled and
monitored by a central computer system, which collects real-time data from the
road network and (in theory) adjusts the traffic signals to suit the
conditions. As you can imagine, these systems
cost a healthy lump cash to install, maintain and monitor. SCOOT is a signal coordination system based
in the UK and according to the SCOOT website, the system offers reductions in
delays of around 12%. So a lot of time,
money, energy and infrastructure are invested in gaining a 12% reduction in
traffic delay.
Consider then, the impact of one vehicle blocking the
movements of an intersecting road for anything up to the entire green
phase. Under a coordinated system, the
length of each signal phase attempts to clear the vehicle queue within a single
phase. Not always possible of course,
but a worthy challenge. Therefore, even
a slight reduction in the number of vehicles able to pass through the
intersection during a green phase has a dramatic knock-on effect on the length
of the queue, and the longer the queue the more green time required to clear
it. In tight inner city road networks,
the queue is more likely to affect upstream intersections, which in turn
increases the risk of another driver stopping in that intersection and so the
ripples spread. Suddenly all that
investment (read tax payers dollars) to achieve a 12% gain in capacity is lost
because of a very small proportion of road users.
Another way to look at this is to invest a fraction of
the cost of a coordinated signal system on educating drivers of the impact they
have and to actually enforce the Road Rule which is in place to discourage this
activity. There are physical measures
available too, including the painted yellow box system.
Comments encouraged!