Central Reservation Barriers

Seth Brundle

What are the best central barriers for dual carriageways? I think the governments idea of using chicken wire with a scraggy hedge has finally been found to be insufficient.
Now, motorways are being given strands of wire. Are these good enough to stop a large artic speeding at 80mph? What happens if a biker hits it? Is it really the 'cheese wire' as claimed?
However, bikers are against armco barriers as these are just like concrete walls to a biker. But these will stop or significantly slow down that artic!

Given that too many people have died here because of bad motorway design (and poor driving!) what should we use?

impr0v

Loads on this on the soc --> commuting/transport board, it's a favourite subject over there.

Monasterevin Bypass central barrier looks very dangerous!!!

M1 crash

BrianD3

Well, the only barrier that is guaranteed to stop an artic crossover is a solid concrete wall about 7 feet high and 2 feet thick. Jersey barriers (small portable concrete things) and normal armco are pretty much useless for stopping a truck. Wire rope seems to work slightly better from the tests I've seen however I would worry about the very low height of some of the wire barriers being installed. Eg the ones on the M4 Leixlip bypass are very low, the ones on the N4 Mullingar bypass are quite a bit higher.

The solid wall idea would probably be completely impractical and too expensive however IMO it would be the best barrier available. Maybe put some impact absorbing material (something like the water filled barriers you see in the states) in front of it to reduce deceleration when hit by a car or motorbike and to reduce bounceback. Crossovers would be eliminated completely and there would be no cheesegrater effect. Put a heavy steel gate in it every couple of miles to allow emergency services to cross if needed. Also make the wall high enough so that it acts as a glare barrier.

BrianD3

mike65

My jokey solution is a 50 metre median with a very soft shallow trench filled with water running for about 30 metres along the centre line of the median and deep sand 10 metres either side.

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Sand

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Trench

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Sand
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You might drown but you would'nt kill anyone else.

Mike.

impr0v

There would also be a lot less dual carriageways/motorways built because of the cost of the increased landtake, so people would continue killing each other on the standard single lane roads.

mike65

T'was a joke...(theres plenty of land in truth just not enough commen sense)

Mike.

impr0v

'tis thin on the ground indeed. How about a 5m thick and high wall of bound straw bales. It would stimulate the waning agricultural sector and eventually decompose into a nature reserve for vermin and birds, satisfying the eco-warriors into the bargain.

sweeper4

mike65 wrote:

My jokey solution is a 50 metre median with a very soft shallow trench filled with water running for about 30 metres along the centre line of the median and deep sand 10 metres either side.

__________________________________________________________

Sand

---

Trench

---

Sand
__________________________________________________________

You might drown but you would'nt kill anyone else.

Mike.

While the Sand and Water is overkill - this does have some mertis! I have noticed while driving in the US that on some of their two lane highways where there is not a concrete median, there is a trench in the centre. The embankments are steepish - the idea I suspect is that if you drift into it at high speed you can't get out, or more importantly crossover. You would have to slow right down again to aim yourself back onto the highway. I would still favour the barrier strategy on dual carrigeway/motorway, but I feel if the NRA were to be taken seriously on their strategy of a wide barrier-free grass median so you can regain control, they would have more support in measures such as that.

The current line that it should be possible to regain control on a 15ft wide strip of grass (Often wet!) is optimistic to say the least.

impr0v

The American solution which you mention is what the NRA were originally implementing with the 7m median, just adapted to an Irish situation. They basically tried to minimise the land provided space for vehicles to shed speed or for drivers to regain control of them, because of the land price differential between here and the US. While it is a hot topic at the moment, historically the 7m strip has worked relatively well. The only problem with it is that when there is an accident involving a vehicle crossing the central reservation, it tends to be a pretty bad one. One particularly bad aspect of these accidents is that the person or person hit by the car coming across from the opposite carraigeway has almost zero time to respond to the developing situation and is completely in the hands of chance, whereas vehicles on the other side have theorectically some chance to react if they are reading the situation ahead of them and maintaining a suitable distance.

Personally I think that these wire rope barriers will do a pretty good job. One particular section of the Mullingar Bypass has been hit twice since installation, once by a HGV, at relatively slow speeds, and once late at night by a car at very high speed. It performed well on both occassions, and neither vehicle crossed the median. The car in fact was deflected back into the carraigeway and hit the barrier twice more before it came to a halt.

sweeper4

impr0v wrote:

The American solution which you mention is what the NRA were originally implementing with the 7m median, just adapted to an Irish situation. They basically tried to minimise the land provided space for vehicles to shed speed or for drivers to regain control of them, because of the land price differential between here and the US. While it is a hot topic at the moment, historically the 7m strip has worked relatively well.

Maybe I should have been clearer. The US median I am speaking of is no wider than the NRA 7m strategy you mention so the land price differential is not part of this argument. The point I am making is that I think they were more realistic by hollowing out the median into a trench to "trap" crossovers. I believe the NRA's strategy would not have come in for as much criticism if they had used similar methods.

seamus

How about a 30 degree ramp along the center of th ecarraigeway, topped with a concrete barrier, like this:

               |
              / \ 
             /   \

Except obviously with a less steep climb. By design, it naturally deflects traffic back onto the carriageway, without serious damage to the vehicle (it should be able to mount the ramp with maybe just a scratched/damaged bumper), and if shallow enough (30 is arbitrary) should allow the driver to regain control. It would be biker friendly, since they'd mount it no problem, and naturally come back on stream, and the concrete barrier can deflect any subborn vehicles who insist on trying to cross over.

For added effect, the ramps could be separated by a 2m trench, in the event that a vehicle does break through one barrier, it'll get hit the second barrier and drop into the trench...

Opinions? (Yes, I'm aware it would be multiple times more expensive than any other barrier.)