how traffic jams start

smemon

http://www.physorg.com/news117283969.html

i'm pretty sure i seen something like this on Top Gear a few years ago.. interesting stuff.

basically traffic jams are caused by people overreacting and braking harder than is necessary.

i have to say, i HATE drivers who are trigger happy on the brakes.. i.e flashes of brake lights every few seconds and going around every corner..

i'll never understand why people won't just sit at a steady speed rather than ping-pong back and forth behind a car accelerating and braking.

i hardly ever use the brakes myself when cruising at speed and will 'absorb' a reduced speed in front of me by simply taking my foot off the accelerator :cool:

astraboy

Interesting. I noticed driving in the US a lot of "Phantom traffic jams" on highways, basically a slowdown and eventually stop of cars on the highway from people a mile up the road braking too hard! People further back end up coming to a standstill for a few seconds. Its not to do with the volume of traffic really, in the experiences I have its due to normal traffic volumes and over reactive driving. Of course we don't have motorways the size of the highways so I'm not sure if its relevant.

Orange69

smemon wrote: »

i hardly ever use the brakes myself when cruising at speed and will 'absorb' a reduced speed in front of me by simply taking my foot off the accelerator :cool:

I do the same, it saves petrol and your discs.. But i think that traffic jams in Ireland are started with incompetent decisions made by the city/county planners..

With a well planned road network there should be no gridlock..

amacachi

I think that there's just something on the motorways that makes brake lights come on more often and prevents indicators from working.

Really though, I always have a little smirk to myself when i see a line of traffic on the M1 moving with a constant speed of about 40-50 mph and one person is constantly on the brake lights but no-one else is, and he/she somehow stays the same distance ahead and behind of the vehicle near them. Of course sometimes the person behind does react and the chain reastion starts.

Balfa

astraboy wrote: »

Its not to do with the volume of traffic really

It's absolutely to do with the volume of traffic. If the freeway is at capacity (in an ideal situation, everyone's doing the speed limit and leaving the minimum safe distance between each other) and driver A enters from an on-ramp in front of driver B, A cuts the distance between driver B and the car in front in half. Driver B now no longer has the minimum safe distance, so they slow down to regain that comfort space. The car behind them has to slow down too, but does so marginally later due to reaction times. The car after that even slower again.

It's like a pyramid scheme... It can't keep going on forever. Eventually someone's so late in slowing that they have to stomp the brakes and almost come to a standstill. The person behind them DOES come to a standstill, and the rest is history...

That's the ideal situation, of course. Normally driver A merges, driver B slows down, and driver C who's an idiot riding B's bumper stomps the brakes and the traffic jam happens much sooner Let's throw in some lorries too, which have an awfully hard time getting back up to 60mph.

If you take the same situation, but halve the number of cars on the road, everyone has twice the minimum safe distance. Driver A merges into the expanse of space between B and the guy in front. B probably doesn't even need to slow down, and A's extra car length plus safe distance is just absorbed into the freeway. Worst case scenario, driver B has to slow down or change lanes, but the cars behind still have twice the minimum safe distance, so they don't have to slow to absorb B's manouvre.

In the article, it says "Drivers and policy-makers have not previously known why jams like this occur". This is complete bollix. It's not something that the University of Exeter has suddenly solved. It's known to every traffic engineer and mathematically thinking person for the last half century or more.

Drik Helbing is an expert on the topic, and he made this nifty simulator that you can play with:
http://vwisb7.vkw.tu-dresden.de/~treiber/MicroApplet/index.html

Try it with the default settings first and witness the carnage. Then halve the vehicles per hour to 1750 and see what happens

Balfa

And by the way, the worst thing is rubberneckers. If there's an accident on the other side of the median, then its only human nature* to have a peek. They stare at it, Once they're past it, they look forward continue driving. But the person behind is still looking at it. By the time they look forward, the car in front is already a considerable distance ahead, and they speed up. But the person behind that is still looking at this point, and by the time... etc.
While staring off to the side, the natural tendancy is to lift off a bit, because who knows what's going to happen in front of you. Eventually this lifting off results in traffic passing the wreck at 40mph, but immediately speeding up to 60mph. The problem is, a freeway moving at 40mph has much less capacity than one moving at 60, and so capacity is exceeded and the situation in my previous post occurs.


* Through evolution I have overcome this flaw in humans I force myself not to look, and pay attention to the cars ahead. One by one, I watch them all accelerate and the accordion (as martin brundle loves to talk about) expands, by the time the fella in front of me is ready to stop looking and realise he's way too many car-lengths behind, I'm already accelerating and leave no gap at all. Even though I don't really make much difference, every little helps So next time you see an accident, don't look at it!

mike65

Wise words, ocassionaly you hear a traffic report on fivelive which notes a crash was cleared but rubberneckers on the opposite carraigeway have bought traffic going the other direction to a standstill.

Mike.

rebel.ranter

Speaking of rubber neckers, a few weeks ago on Cork's South ring (on my commute) there was an accident goign East (An Alfa 156 was facing the wrong way on the verge as a result). But to make matters worse the clowns on the West bound carriageway ended up in a 4 car pile up when they were having a look at the carnage on the other side of the road!!

bman

Balfa wrote: »

Drik Helbing is an expert on the topic, and he made this nifty simulator that you can play with:
http://vwisb7.vkw.tu-dresden.de/~treiber/MicroApplet/index.html

Try it with the default settings first and witness the carnage. Then halve the vehicles per hour to 1750 and see what happens

Just what I needed! I'm hungover and severely lacking in sleep. This should keep me amused for about 1-2 hours!