Comment by adrian_b | Hacker News Reader

Comment by adrian_b | Hacker News Reader

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Other countries use a much cheaper method.

At some distance before the tunnel or passage under a bridge with limited height (but after the signs that advertise this) there is a solid metal barrier at the height limit and when any too high truck attempts to go forward (which of course happens from time to time) the barrier cuts the top of the truck, which is guaranteed to attract the attention of the drivers (and presumably the anger of their employer).

EDIT: I have posted this before watching the video, which shows exactly how this method, of using a steel "crash beam" works.

In this case, the truck was only slightly above the height limit, but in another place I have seen a similar incident where the truck was higher so all its roof has been cleanly cut and detached from the truck.

This is the right method of handling such height limits, the only one that is guaranteed to be foolproof.

Even if it damages the vehicle, if the vehicle were able to pass with an extra height, it is certain that it would be damaged anyway, but in worse conditions, especially in the case of a tunnel.

> This is the right method of handling such height limits, the only one that is guaranteed to be foolproof.

Near where I grew up, there was a low bridge. At one point many years ago, an excavator on a low-loader hit the bridge, causing the excavator arm to swing around... into the upper deck of a loaded bus travelling in the opposite direction.

The point is, this method might be foolproof in terms of preventing damage to the bridge, but it has other potential drawbacks.

> I have posted this before watching the video

They've done every conceivable thing to mitigate the problem at this bridge. Watch the old videos, before they had the crash bars and huge signs and flashing lights. Before they raised the bridge to the limit of what the rail line could tolerate.

> This is the right method of handling such height limits, the only one that is guaranteed to be foolproof.

Well, it probably causes a lot more risk to life than the water curtain one.

To be fair, it's the driver that's causing the risk to life, not the barrier.

It makes sense to provide plenty of warnings and if a driver isn't paying attention, then they need to be stopped before they damage something really expensive.

What is the industry standard threshold of expensive damage where an engineer can decide to kill a person to avoid it? I imagine this comes up often enough there would be some sort of formal guidance about it.

My perhaps naive understanding of civil engineering professional ethics is that this decision is off limits to them. They may design a system to fail in ways that are less harmful to life than others, but not to cause human harm or death when it could otherwise be avoided.

In any case, if you do design such a system, the driver may be at proximate fault but it's hard to say it's entirely on them.

Most objects aren't designed to withstand impacts from heavy machinery (e.g. lorries) without endangering anyone and it's not normally considered. Shop windows can cause significant damage to people if a vehicle is driven into them, but it's not really anything to do with the ethics of window designers/manufacturers/installers.

If there was a large pointy protrusion that was designed to cause harm, then you would have a stronger point(!) that it was designed to be dangerous.

Merely having a height barrier before the very similar height barrier of the tunnel itself is hardly making anything more dangerous and I'd consider it less likely to cause harm to life as emergency responders can get better access to the people involved rather than possibly having to dig them out of a tunnel first.

Also, a height barrier is unlikely to cause major physical harm to the driver, so I think you've over-egged your position.

Right but once you've decided to intervene to stop the driver from damaging the bridge, the mechanism of stoppage is your responsibility. The way you stop them may not be, and doesn't have to be, safer than what would happen without intervention. But at the same time you can't go "eh it's cheaper just to kill them."

Hitting the height barrier is likely to be less harmful to the driver and other people than hitting the bridge itself, so that seems to be taking due care (and again, killing the driver is extremely unlikely in either scenario).

The purpose of a height barrier is to act as a sacrificial component and I would consider that it improves the safety of the structure as well as being cheaper. It's a very common design to see metal or concrete poles (bollards) used to prevent drivers from causing harm to other road users, features or shop fronts. They're safe and effective.

Near where I live we've got a guided busway (a track system that modified buses can travel down, tram-style).

On the railway bridge before it's entrance there's a height barrier in advance consisting of a whole load of bells that dangle down from a metal bar and will hit over-height vehicles without making a really solid collision.

They still haven't figured out a way to stop cars driving up the bus tracks, though (where they inevitably get stuck).

Yeah, or just some metal pipes on a string; if the vehicle hits it, it'll cause a racket but no significant damage.

Those were already there, the trucks ignored them...

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