Comment by fransje26 | Hacker News Reader

Comment by fransje26 | Hacker News Reader

Parent Full thread

As the credited inventors of the modern roundabout, I've always wondered how the UK traffic planners managed to misunderstand their own creation to the point that they come up with such designs..

An other baffling design I've encountered in the UK is a roundabout with traffic lights half-way through.. Wasn't the concept based on removing traffic lights to fluidify traffic..?

As a whole, if designers come up with far-fetched designs where drivers struggle to understand what's going on, they are doing something very wrong. Assuming the average driver is already barely in control (phone distraction, screaming kids, lack of sleep, medication, subpar vehicle control, etc), the last thing you want to do is remove even more situational awareness by coming up with over-complicated designs that require serious thinking.

The argument that people will slow down because they don't understand what is happening is a fallacious one. Yes, they will slow down, but then, under stress, they will probably default to some instinctive basic reaction which has a high probability of being incorrect, leading to accidents. An illustrative example can be made with traffic videos of American roundabouts.

https://www.youtube.com/watch?v=Xu0VJ7Vo5pg

So in conclusion: Keep It Simple Stupid. (KISS)

> An other baffling design I've encountered in the UK is a roundabout with traffic lights half-way through.. Wasn't the concept based on removing traffic lights to fluidify traffic..?

This is indeed a weird one from a US perspective.

The way to think about it is not as a roundabout with traffic lights, but as a light-controlled intersection in the shape of a roundabout.

A roundabout-shaped intersection can handle more variations than a normal intersection, you can have more than 4 roads, or roads entering at odd angles.

As for what the advantage is of having lights on the roundabout as opposed to on the approach, I have no idea.

Roundabouts with traffic lights are a bit of a failure alright - and the larger ones are i'd argue not roundabouts at all - they just share the shape.

I believe traffic lights are always trying to solve a capacity issue - where the roundabout has hit it's maximum capacity and is suffering some throughput issue, which tend to sort of get exponentially worse. With traffic light sequencing, particularily dynamically, there is always a way to even out the flow - prioritize a flow that is backing up undesirable or give a particular entrance fair chance to enter the roundabout.

Though once there are traffic lights on every entrance, plus traffic lights mid roundabout and some/all exits, and explict lane markings and merges I think it's not a roundabout.

Slowing down is important though, as it give drivers time to think and react. Whether they choose to use that time correctly is a problem, but hopefully some or all of the other drivers can use patience and avoid an accident. Where accidents happen, I see it's often from mistakes from two drivers, and it's relatively low speed. Better still, accidents are at shallower angles, so injuries are rare. I've heard an statistic that could well be fake that roundabouts have more accidents, but significantly better outcomes overall.

The issue that can happen is when one direction has significantly more flow than the other - in this case you end up needing traffic lights otherwise nobody can ever pull out from the minor direction at rush hour.

The other issue in the UK is massive signalised roundabouts used for junctions where traffic volumes clearly justify a proper grade separated junction like a stack, purely to penny pinch. South Mimms A1M/M25 junction is a good example, or the M2/M25 junction where they eventually had to put in free flowing slips eastbound to northbound because the roundabout was constantly congested.

That's not really a problem with roundabouts per se though, it's just bad design choosing an inappropriate junction design purely to avoid having to pay for bridges

Bridges take a lot of space (to make a gentle enough slope) and I think in many cases in Britain, that space isn't available.

Fixing one junction might just move the congestion to the next one, and if you continue fixing them you turn all the cities into jungles of concrete.

> Slowing down is important though, as it give drivers time to think and react. Whether they choose to use that time correctly is a problem, but hopefully some or all of the other drivers can use patience and avoid an accident.

I fully agree, and I also think that the "original" roundabout design serves that purpose well, although throughput might different than with dynamic traffic lights.

The point I was trying to make was that slowing down traffic through added complexity could be a dangerous approach to take. It's a switch from a low cognitive load approach by simple design -slow down to a stop/almost stop, look to one side, give way if necessary-, to a slow down of higher cognitive load -slow down, figure out how to navigate a more complex (new) intersection, and maybe remember to give way-. So where in the first approach cognitive load is used to assess how to give way, in the second approach some of that load is used to deal with a more complex/unfamiliar situation. And for some users, I argue, that could already push them more towards accident territory as less cognitive capacity is available to properly assess the traffic situation. Sometimes less is more.

Traffic lights are sort of a 'patch' on them. They usually start off without them, they realise theres flow issues that prevent people coming out of one junction, and its going to either be too costly or impractical (space is a massive issue in the uk remember) to switch to something more complex, so lights are used as a permanent stopgap.

One example of where this has been an ongoing issue for decades is the Black Cat Roundabout on the A1 (https://en.wikipedia.org/wiki/Black_Cat_Roundabout). It's gone through numerous changes to improve things, with lights at various points being one of them. Finally now though its being completely redeveloped to a grade-separated junction as traffic has massively outgrown the roundabout.

Reply on news.ycombinator.com