Introducing the Squircle

Image taken from An Application of Safe System Approach to Intersections in the Capital Region - Pilot Project. Phase 1: Progress Report.

Image taken from An Application of Safe System Approach to Intersections in the Capital Region – Pilot Project. Phase 1: Progress Report.

Ladies and gentlemen allow me to introduce you to the squircle (click on the image to the left to enlarge it). This is an intersection concept that I first came across a couple of years ago but had completely forgot about until seeing it at a presentation I attended on Wednesday. The idea is basically a combination of a signalized intersection and a roundabout. I think the name could use some work, but don’t worry, it’s not nearly as complicated as it looks.

There is a lot going on in that photo, so allow me to explain. If you want to turn left or right nothing changes from the standard signalized intersection. The only difference is on the through movements, which now have to deflect slightly as they enter the intersection, much like what you’d see at a roundabout. The lighter grey areas in the middle would raised slightly to make sure drivers end up in the right place. So like I said, it’s not nearly as complicated as it looks. But now that I’ve explained it you might be wondering what the point of all this is if nothing really changes.

Well, nothing much changes for the drivers using the intersection, but this minor tweak could result in a much safer intersection. There are two reasons for this. The first is speed. If you were to drive through this intersection the geometry forces the driver to slow down. Regardless of the design effort put into an intersection there will always be collisions, it’s unavoidable, but if the vehicles involved in a collision are moving at a slower speed those collisions will be less severe. That’s one of the ideas behind this intersection concept.

The second is the angle that a vehicle turning left and another traveling through the intersection would meet, this design flattens that angle; the 90°, or t-bone, collision is more or less eliminated. In the City of Edmonton’s 2012 Collision Report “left turn across path” is identified as the second leading collision cause that is likely to result in fatality or injury with 11.5% of collisions resulting in a serious injury or death. This design doesn’t eliminate the left turn so there will still be collisions but by changing the angle we can, once again, reduce the severity of the collisions. And if the speed is also reduced then the benefits could be substantial.

There are trade offs though, the biggest being capacity. If vehicles are travelling at slower speeds then fewer will be getting through the intersection during each phase. If you were looking to retrofit an intersection with this design it would, in all likelihood, be because the intersection had been identified as a problem location with a higher than expected number of collisions. In that case I doubt the reduction in capacity would be a significant factor in the decision to make changes but it shouldn’t be ignored entirely. Another issue with this design, at least here in Edmonton, is snow removal. Clearing snow from the centre of an intersection like this would take a lot more time and attention to detail than clearing snow is required at a standard signalized intersection.

Personally, I’d rather see a few more roundabouts (much lower collision rates and no signal operation costs), but if space was limited or a roundabout couldn’t support the traffic volumes, I think a design like this could work quite well. It’s certainly worth considering. Something really needs to be done about the name though.

3 thoughts on “Introducing the Squircle

  1. Zsolt Munoz

    So is the squircle a signalled intersection? It would appear so but I’m just wondering.

    1. ryanbatty

      In theory you could use this design at a signalized intersection or at a two-way stop control; there would be no value at a four way stop. However, at a two-way stop there would be little value in deflecting the through movement controlled by the stop sign since that movement would have no possibility of moving through the intersection at a high rate of speed. But it could certainly be done.

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