On Tue, 2 Aug 2005, Earl Purple wrote:

And actually, a road, if used properly, will usually take a greater
volume than a railway. On a D2 dual carriageway, for example, if cars
are travelling at a 2-second gap,

Then they're travelling too close together. Unless the traffic's flowing
at 40 mph, at which speed 2 seconds is almost enough.

I'm basing this on highway code rule 105, which says:



105: Drive at a speed that will allow you to stop well within the distance
you can see to be clear. You should

* leave enough space between you and the vehicle in front so that you
can pull up safely if it suddenly slows down or stops. The safe rule is
never to get closer than the overall stopping distance (see Typical
Stopping Distances diagram below)



The diagram it references gives the following stopping distances for
various speeds:

Speed (mph) Distance (m)

20 12
30 23
40 36
50 53
60 73
70 96

If you divide the distances by the speeds, you get the following times:

Speed (mph) Time (s)

20 1.35
30 1.73
40 2.03
50 2.39
60 2.74
70 3.09

If you're talking about a dual carriageway, then presumably you're hoping
for a speed of 70 mph, which would require a gap slightly longer than 3
seconds. Of course, if you're talking about a road in London, then 40 mph
is probably quite realistic!

you get 30 cars in each lane passing per minute. If each car has 2
occupants,

Then that's very good going. The average occupancy is about 1.5.

that's 120 passengers a minute in each direction.

Or, if you use realistic numbers, 60.

You'd need to run a very frequent train service to carry that many.

Let's go with your number of 120 pax/min, which is 7200 pax/hour.

Let's compare that to a reasonably modern and efficient tube line, the
Central - the nominal capacity of one of its trains is 620 passengers, and
it runs 30 trains per hour, for a capacity of 18600 pax/hour, over two and
a half times that of your motorway, for a fraction of the land take. And
if you have the ability to use 12-car mainline-gauge trains, like
Crossrail, then it's even higher.

tom

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