[Gllug] [OT] Times Article on ABD was Fighting a virus

[John G Walker]

On Tue, 20 Feb 2007 11:55:25 +0000 John Hearns
<john.hearns at streamline-computing.com> wrote:

> John G Walker wrote:
> > 
> > Clearly bus lanes also make a difference. But a moment's thought
> > will tell you that the introduction of bus lanes will also cause
> > congestion in the other lanes (same amount of traffic in a smaller
> > space). 
> 
> Not so. It is a matter of impedance matching (hey - we're veering
> closer to topic here). If there is a hold-up or slow-up further down
> a route then no matter how many lanes you have - ten, twenty, thirty
> then traffic will still come to a grinding halt. All you create is a
> wider car park.

Correct. (And this is just sysadmin theory in different disguise.)

> I will give you a for instance - Lower Road in SE16 is always 
> chock-a-block in the evening rush hour, with cars going south. There
> is a bus lane on the left. Talking away that bus lane would make no 
> difference to congestion - there is a line of cars stretching from
> the Jamaica Road roundabout down to Greenwich. Taking away the bus
> lane would allow one more lane of stationary cars.
> 

I have to accept, since I don't know the road, that taking away the bus
lane would make no practical difference. But that's not the same as no
difference at all.

Remember your queueing theory. The effect of a greater (or lesser)
demand on resources is not linear. And when demand reaches about 80% of
the available resources the rate at which the queue forms increases at
a rapidly ever-increasing rate. Hence the advice to sysadmins to
always ensure that less than 80% of resources are used.

Traffic congestion is, or course, just another form of queue. So all
you're saying here is that taking away the bus lane on Lower Road will
not reduce road usage to less than 80% of capacity. Which I accept.It
doesn't alter my argument.

However, as you point out, there's another factor involved. Increasing
throughput can merely mean that the queue moves up the line:

> And take a look at the famous M4 bus lane. It is indeed a bus lane,
> but its real purpose is an impedance match between the 3 lane section
> of the M4 beyond Heathrow and the 2 lane M4 elevated section. Stops 3
> lanes of traffic piling up at the 2 lane elevated section.
> 
> 


-- 
 All the best,
 John
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