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The crossing in question

I think that just the sheer number of signs should have told the woman that something was up, even if she didn't know that the sign with the gate thingy means "Level crossing with barrier or gate ahead" (according to the Highway Code, which she's supposed to know if she passed her test).

Two signs say, "Drivers of long low vehicles phone before crossing" - even if you realise that the sign doesn't apply to your little toy car, surely you'd ask, "Crossing what?"

Not to mention that there's a complete set of instructions for using the crossing, directly underneath the lights, including the instruction to "open both gates" then cross.

Yes, there are lots of signs, and they could be confusing. That's a pretty good reason to look at them properly, wouldn't you think? I can't say I have much sympathy.

This piece of faulty logic is being bandied about all-over this discussion. You people bad-mouthing those who drive smoothly up to lights are forgetting a major fact: That the faster the traffic is moving the further apart the vehicles drive. If people drive too close, the inevitable concetina of braking occurs down the traffic, but if they drive far enough apart (i.e. properly) then less traffic will get through a junction than slower moving traffic.

Years ago I heard an anecdote (wow, what a way to support my argument) that more traffic can get through spaghetti junction in Birmingham, UK if they are doing 30mph than if they are doing 70mph, if the traffic is driving the correct distance apart [1] ("the 2 second rule").

At 30mph the stopping distance is 75 feet, and at 70mph its 315 feet, so at those speeds your car is essentially its physical length plus the stopping distance. The junctions are always the same size, so less vehicles can safely drive through a that junction at high speed than low speed. The gaps between vehicles only change when people change speed, so gradually coasting up to a red light means the gaps can stay consistent (and potentially less than 2 seconds, and thats more like what really happens).

People who race up to lights, stop, then pull away again are also forgetting about little period of time each person in the queue when the person infront of them takes to pull away. Those times add up quickly.

[1] Feel free to argue that people don't drive the correct distances apart, but people do drive further apart at speed than slowly. The official numbers (UK law) are here if you want to work it out: http://www.highwaycode.gov.uk/09.htm#105)

You're forgetting that the capacity of the road is usually based on how many vehicles can fit along it, ie vehicles/lane-mile - and as speed reduces, there is need for less gap between vehicles and so more vehicles can fit per lane-mile; the flow is the number of vehicles that pass by in a given time. Congestion occurs when there is not enough road space for the vehicles.

Braking distances given in tables are based on square of speed. As speed reduces, so does braking distance. If the stopping distance as given by such tables is left between vehicles, the time between vehicles will also change (eg at 20mph, dist=40ft which is ~1.4secs; at 70mph, dist=315ft, which is ~3.1secs). At just below 40mph (38 2/3mph) the braking distance given by the table actually takes 2 seconds. So above about 40mph, the 2 second separation rule is giving less than the stopping distance between vehicles, but those speeds are [usually] reserved for "fast" roads where something isn't really expected to "jump out" and cause the preceding vehicle to stop instantly, so the thinking and braking distance an overlap the vehice in front - in the time it takes you to react, the preceding vehicle will [usually] have moved forward [relatively] quite a way.

So how does reducing the speed improve the capacity, and flow?

The maximum capacity is when there is zero gap between vehicles, ie their speed is zero, but that is the worst case flow (also zero).

So what about 70mph & 60mph? Assuming a car of about 15' length and a separation of 2 seconds:

70mph: 2 secs ~=205', car-space = 205+15 = 220, giving 24 cars/lane-mile. Thus we have 70 * 24 = 1680 cars/hour.
60mph: 2 secs =176', car-space = 176+15 ~= 190, giving ~28 cars/lane-mile. Thus we have 60 * 28 = 1680 (actually ~1667) cars/hour

So although the speed has been reduced by 15%, the flow has only been reduced by barely 1%; but the capacity of each lane-mile has increased by about 16% - an extra 4 cars are able to use each mile of each lane of the road.

If the stopping distance is left between vehicles: at 70mph, the cs is 315+15=330', allowing 16 c/lm, or 1,120 cars/hour; at 60mph, the cs is 240+15=255', allowing ~21 c/lm, or 1,242 cars/hour. So by reducing the speed by 15%, in this case the capacity has increased by about 29% (5 extra cars/lane-mile) and the flow has increased by almost 11%!

If you do the maths, you'll find the optimum speed for the max flow when the stopping distance is left occurs when speed = sqrt(20 * mean_length_of_vehicle)! So the more longer vehicles there are, the faster the speed for optimum flow - so be grateful for all the coaches and arctics.

From the UK Highway Code which uses the age old formula:

distance = thinking distance + stopping distance = speed + speed^2 / 20

where distance is in feet, speed is in mph.

At 70mph, 2 secs is approx 205ft. According to the UK Highway code, the average car length is 4m, or about 13ft, so leaving 2 secs between cars leads to a front bumper to front bumper distance of 205 + 13 = ~220ft which allows for 5280/220 = 24 cars/lane-mile.

Just remember that the UK has a much smaller area than the US; and our cars are much smaller.

Incidently, when I did my PCV (bus driving) test 12 years ago, these typical stopping distances were the ones I was expected to know for the coach.

From the UK Highway Code which uses the age old formula:

distance = thinking distance + stopping distance = speed + speed^2 / 20

where distance is in feet, speed is in mph.

At 70mph, 2 secs is approx 205ft. According to the UK Highway code, the average car length is 4m, or about 13ft, so leaving 2 secs between cars leads to a front bumper to front bumper distance of 205 + 13 = ~220ft which allows for 5280/220 = 24 cars/lane-mile.

Just remember that the UK has a much smaller area than the US; and our cars are much smaller.

Incidently, when I did my PCV (bus driving) test 12 years ago, these typical stopping distances were the ones I was expected to know for the coach.

I may be wrong, but I vaguely remember reading somewhere that the best flow occurs when the traffic does merge at the pinch point (where one of the lanes physically can no longer be used and a single lane continues), one vehicle from each lane in turn (ideally, as they head towards the pinch point, the vehicles should match speed with each vehicle next to a gap so that a smooth merge is effected: just like a couple of cogged wheels).

With people trying to merge before this point, you'll get many merge points, each with their queues - as the queues merge, the traffic slows, but once merged, it can speed up again; however, it then comes to another merge point and slows again, repeating until the physical pinch point whereupon the 2 streams of traffic have to become one and can then flow properly again. Also, you'll get the effect of "I've been queuing for ages, so I'm not going to let you in", and "I'm not going to let that bus beat me", etc resuling in tailgating and forcing any traffic to have to slow in the dying lane, making it more likey that traffic in the lane that continues will also have to slow down, upsetting the flow.

This is like a stretch of two lane highway which is reduced to one lane by two (or more) sets of road works, with no exits between them: there will be a queue at the first set where the capacity of the road is reduced to 1/2 (two lanes to one), but there will also be another queue at the next set, even though the capacity through all the road works is the same - one lane. Trying to merge before the physical pinch point is a bit like creating many "temporary" pinch points - each will cause a queue, somtimes with a bit of reasonable flow between them.

The basic problem is one of roads running near (if not over) capacity - it is an unstable flow: as long as the traffic is flowing, it will flow, but as soon as there is a slight hiccup, the flow breaks down and you get the phantom jams (the hiccup doesn't have to be as dramatic as an idiot suddenly changing lanes, it can be a simple small lapse in concentration effecting an over reaction creating a shock wave that runs backwards through the traffic, eventually causing someone to actually stop); and selfish behaviour by drivers, "racing".

As an aside, in the UK Highway Code it lists the typical stopping distances for various speeds. Using the figures given, to allow the maximum amount of traffic to flow along a stretch of road, if everyone kept these distaces between them and the vehicle in front, the optimum speed is given by sqrt(20 * len) mph where len is the average vehicle length in feet - thus if you want to get the most amount of traffic to flow at the highest speed possible, you want quite a few (ie a lot of) long vehicles (buses, lorries, etc)!

I just read the Highway Code at http://www.highwaycode.gov.uk/index.htm

There is no reference that I could find to braking as a resolution to tailgating. I don't know if you're suggesting that it condones hard breaking, or a light application of brakes in order to gradually reduce speed. Panic breaking for no reason is certainly illegal is Washington State, and in the UK it's probably covered under the careless or inconsiderate driving law, which carries a GBP2500 fine. It also runs the risk of the person not reacting in time and slamming in to you. No amount of personal satisfaction should be worth the risk of causing a serious accident.

I used to get a lift to work and the guy who drove me swore blind the middle and left lanes you could just sit there and the outside lane was the 'overtaking lane'. He wouldn't accept me telling him that actually EVERY lane except the left is overtaking (depending on how many lanes you have).

And some links if people are interested.
Three lane dual carriageway
Some Motorway information
Road signs, if anyone cares :)

I used to get a lift to work and the guy who drove me swore blind the middle and left lanes you could just sit there and the outside lane was the 'overtaking lane'. He wouldn't accept me telling him that actually EVERY lane except the left is overtaking (depending on how many lanes you have).

And some links if people are interested.
Three lane dual carriageway
Some Motorway information
Road signs, if anyone cares :)

I used to get a lift to work and the guy who drove me swore blind the middle and left lanes you could just sit there and the outside lane was the 'overtaking lane'. He wouldn't accept me telling him that actually EVERY lane except the left is overtaking (depending on how many lanes you have).

And some links if people are interested.
Three lane dual carriageway
Some Motorway information
Road signs, if anyone cares :)

The Highway Code states that the sign is a "Temporary maximum speed limit".

"I couldn't find the freeway and had to take some little back road called the M5"

Green A roads are Trunk route and maintainance is paid by the Highways agency (I think) rather than the local councils.

More on the speed (only for cars lorrys, busses, caravans etc. have lower limits). Road type doesn't matter.

Motorways are always 70mph unless signposted.

The Speed limit in urban areas (having street lighting not more than 200 yards apart) is 30mph unless signposted.

Outside urban areas:

Dual Carrageways are 70mph, Single Carriageways 60mph. Lower speed limits must be signposted and repeated every 200 yards (with small signs)

But there are so many signs everywhere in the UK now a speed limit sign will be near...

More here

How about this four lane roundabout then: http://www.cbrd.co.uk/badjunctions/45-46.shtml

It's actually not a bad roundabout when you know which lane you want to be in, but for visitors to the area it must be a nightmare.

And here's some info on how to use a roundabout, from the British Highway Code:
http://www.highwaycode.gov.uk/17.shtml#160

The Highway Code (a bunch of guidelines, some of which have legal backing) suggest that motorists give way to them, but there doesn't seem to be any specific offence that someone who fails to let an emergency vehicle pass can be charged with.

As the other replier alluded, you need to watch the National Lampoons. If you want British instructions for dealing with them, take a look at the relevant section of the Highway Code.

However, a quick look through the Highway Code (2001 edition) (a government book giving a guide to UK traffic law) showed only the following under rule 128: ".....PCs, multi-media, etc. Do not operate, adjust or view any such system if it will distract your attention while you are driving;...", so perhaps the rules have now changed to "You can have a TV provided you don't use it when moving". Can anyone find a link to the relevant legislation?