h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Why? Travelling at 30 mph does not make my car any lighter and exposes me to the risk of falling through the bridge for a greater length of time.

PP

This has always intrugued me too. Near where I live a major bridge over the motorway had a sign saying "Caution - Weak Bridge" (it didn't have a reduced speed limit on it). Surely either the bridge is strong enough to take the traffic or it isn't - is the warning a form of disclaimer, so that if the bridge collapses, they can't be held responsible???

Personally I would rather put my foot down, take a good run at it and hope that if the bridge collapses the car will take off like the General Lee in the Dukes of Hazzard!

EeeeeHaaaa!

Physics was never my strong subject, but maybe the reasoning goes like this. Traffic sets up vibrations in the road. (Trucks going past my parents house used to make the windows vibrate, and cars in the driveway next to our house do the same to our door glass.) These vibrations may cause structural problems in an already weak bridge. At high speeds the vibrations are stronger than at low speeds, so crossing the bridge slowly will do less damage than crossing it quickly, hence making it less likely to collapse. I'm guessing, but it's the only thing I can come up with.

Yeh, like the Millenium Bridge in London - they said people were 'walking the wrong way' across it and set up massive vibrations/wobbles - I think they were probably just trying to get off it a.s.a.p. before the whole thing collapsed!!

(By the way, what are the chances of the Bridge re-opening before the next Millenium???)

It is a very pretty bridge to look at though.

But it blocks the only clear view of St Pauls from the river. Perhaps Someone was getting Their own back on the planners

That's the problem. It's very pretty, but the designers forgot that it was supposed to be a bridge rather than just a decorative feature. It rather reminds me of all those house makeover programs (i.e. changing rooms etc.) they concentrate on making the rooms 'look' attractive, forgetting that they are supposed to be of some practical use.

On the subject of weak bridges, last week I heard on the traffic news that there was a speed limit on a motorway (up North somewhere - cant' remember which one), where they had imposed a speed limit as the road was sinking and they expected it to sink up to 5 ft.(???) over the next few weeks. Did anyone else hear this? Did I hear it right?

Yes, I heard that one too - I remember something about a road sinking 5 feet, but I can't remember the details - I wasn't listening properly and by the time it had registered with me it was too late.

I think that the motorway in question was the M6. If not it was another up there where the rainbows are in black and white.

PP

Though the mass of your car does not alter, the weight of your car does increase with speed. This is particularly amplified if its a humped bridge design. But it's vibration etc that are the killers i think.

The road in question is the M6, but the stretch is not 'up North' unless you count Birmingham / Stafford as 'up North'. Round here, it's the Midlands. The road has already dropped 4 feet, but a concrete section was inserted a while ago to help counteract the worst of the problem. The various traffic agencies reckon that drivers should only feel the undulations if they exceed the recommended speed of 50mph. I know the M6, and there are times when you're lucky to go as fast as 50mph!

Those southerners! They think north starts at Watford . Near Birmingham sometimes you're lucky if you can go 5mph!

Einstein would have been interested to hear that weight increases with speed but that mass doesn't. Can you explain this, please?

I think momentum is what was meant

Yeah, but that would be forward momentum. I don't see how that would impact on the bridge.

Sorry, I live in Gloucestershire, so anything North of Brum is (geographically) up North!!

Brum is Midlands to Surrey inhabitants. But then we will travel to Worcester and back in a day on business.

As far as I'm concerend the EeeeeHaaaaa method is still favourite!

I'm in Manchester, which is seen as Oop North by those in the south (which starts south of about Northampton, in my view) but is generally described as the North West. North of the North West is the North. But on the other side of the Pennines, in Yorkshire, they are in the North, and north of them is the North East.

So, if you are following this (good grief, have you nothing better to do then read my drivel? ) the North is north of the North West, but south of the North East. Does that help?

The whole thing is daft when you consider how tiny our counrty is compared to Oz or US.

What it really comes down to is whether you can afford the petrol in the first place! I have to find £36 to fill my car's tank up!

Well, it's all that EeeeeHaaaa. Your petrol will take you further at 30mph.