Sunday, 13 January 2019

Crash Friendly

We are concerned that nobody is hurt on our streets and roads as a result of a mistake by them or others and this underpins a better approach to highway design. Of course, when someone gets it wrong, they shouldn't pay for it with their life.

Despite the promises of self-driving vehicles always being ten years away, we have to live in the now and that means we have to install 'stuff' into the highway to warn/ inform/ regulate people and that means traffic signs, bollards and the other kit used to manage the flow of people. The trouble is, people do make mistakes and the kit tends to be crashed into. This could be anything from the almost comedic person walking into a lighting column because they were distracted, to a driver on a high speed road having a blow-out and crashing into a bridge pier.

Perhaps we can think about energy. On the one hand, walking into a lighting column can hurt, but hopefully, you wouldn't be too damaged; although some people are less able to recover. On the other hand, crashing a car at 70mph into a bridge pier is almost certainly going to kill everyone in the vehicle. In other words, the greater the energy involved, the greater the risk.

For the lighting column example or indeed any piece of street furniture, we should take care with placement so that we don't leave it in a desire where someone might fall over it or walk into it; visually impaired people are going to be disproportionately affected. For the bridge pier example, we are going to have to put in some sort of protection which will absorb the energy of the vehicle and either stop it hitting the pier or divert it away.

One feature of our roads (motorways and trunk roads in this case) are traffic signs. Because of the speed of traffic, the text on the signs is large and so the signs are also large. Large signs present themselves to the wind and so they end up being attached to chunky posts with substantial foundations. You really don't want to crash into one of these large posts as the car tends to wrap itself around the post.

In some situations, safety fence (popularly known as crash barrier) is rolled out. You've all seen the type of thing, here's a video of it being installed;

The system uses posts installed into the ground with the barrier attached. In the event of a crash, the barrier is designed to catch the vehicle and absorb the energy. Some barrier systems are tensioned to further help with absorption. People often talk about wanting to see safety fencing along footways and cycle tracks next to main roads, but this doesn't mean that they'll be kept safe as this crash test shows;

The issue is that in order to absorb energy, the posts are going to snap and the barrier will buckle and stretch - you don't want to be standing behind it. This is also the reason that if you break down on a motorway, don't sit on the barrier waiting for help - try and get away from the road as far as you can safely go. Safety fence design considers this movement called the 'working width' and so any paths behind need to be set back beyond this working width. Of course, getting people away from the very big roads would be a better answer, if not always practical.

The problem is, we cannot put safety fence everywhere. It's costly, it requires maintenance and replacement and in some places, it's just not practical because of space or other issues. We can take a risk approach and so where we have bridges and gantry signs, safety fence is useful. For large signs, we have other options - passive safety posts.

The photo above shows a very large sign next the A13 in London where the speed limit is posted at 50mph. A sign of this size needs some chunky posts to hold it up, but the available space means it's quite close to the road and in the event someone loses control, they could well crash into the sign and its posts. The designer has chosen to use 'Lattix' posts which are strong in use, but in the event of an impact, they crumple;

We also have other things to contend with such as lighting columns. On trunk roads and motorways, they tend to be quite large and again, it's not practical to have safety fence in front of all of them. A risk based approach can be taken and as with sign posts, there are energy absorbing lighting columns which fold and buckle as they absorb the energy;

Motorways and perhaps to a lesser extent, trunk roads, are very consistent in layout and opposing traffic flows are physically separated. This hasn't happened by coincidence, it is the result of work done to develop standards for high speed situations which are mandatory and require sign-off processes to depart from those standards. Of course, once you throw people into the mix, things can be a little unpredictable and very dangerous for the people working there;


  1. Hmm, guess I can't count on those lamp posts to protect me from wayward drivers on the pavement.

    1. Indeed - people are surprised at how flimsy they are!