"Mini-roundabouts": emergence of a virtuous circle

"Mini-roundabouts": emergence of a virtuous circle

Most of us are familiar with a roundabout, that is, a small circular intersection where we yield upon entry and then travel counter-clockwise to our preferred exit.

The potential advantages of roundabouts - particularly one lane roundabouts - are numerous, including:

  • Reduces off-peak delay compared with traffic signals
  • Dramatically reduces major crashes compared with traffic signals
  • Lower operating costs than traffic signals
  • Generally far more aesthetically pleasing and can create a "gateway" effect

However, roundabouts can have several limitations, including:

  • Capacity - One lane roundabouts are generally not suited for high-volume conditions.
  • Safety - Two lane roundabouts, while able to handle higher volumes - have a mixed safety record.
  • Cost - Both one and two lane roundabouts typically cost far more than a traffic signal, with an average cost of around $800,000 for a one-lane, full-size roundabout in North Carolina.

The first two challenges have led me to generally favor other options like "Synchronized Streets" - also known as "superstreets" - for higher traffic volume intersections.

For moderate volumes, a one-lane roundabout can work well and safely, and the capacity limitation can typically be addressed. But, the third bullet above, cost, has remained a challenge.

It turns out that there is a way to address the cost issue, at least for lower-speed roadways with more moderate demands - let's say up to 15,000 vehicles entering the intersection throughout the day.

A "mini-roundabout" could reduce traveler delay and increase safety at intersections with low speeds, at a cost comparable to a traffic signal.

I recently participated in a webinar by the Transportation Research Board on the potential applications of mini-roundabouts which highlighted several benefits.

Mini-roundabouts are small - with the entire diameter of the outer curb of only 90' or less, perhaps as low as 50'. The smallness is a design feature, creating a "virtuous circle" for this circular intersection.

The smaller "mini-roundabout" will take up less space, require less land and right-of-way, and be less disruptive to the landscape. Since it can fit in smaller, existing spaces, it can be constructed more quickly, which means it will also be less disruptive to travel and commerce, and require fewer utility relocations.

Even better, the smaller size and simpler design means that a mini-roundabout is also less disruptive to the pocketbook:  one can be built for around $300,000, which puts the mini-roundabout in the game compared with a typical traffic signal installation.

What's more, the smaller radius helps keeps speeds in the mini-roundabout as slow as 15 MPH, which should make travel easier for bicycles.

As with any solution, mini-roundabouts are not the answer everywhere, because they simply will not work everywhere - they have a lower maximum capacity than a standard roundabout, let alone an efficient traffic signal.

In addition, the jury is still out on their safety record when applied to higher speed roadways - while they may still have application in those situations under certain conditions, the experience is still limited right now.

But there are several mini-roundabouts on lower-speed roadways in the US now — like this one in Harford County, MD — with  with more on the way. For lower-volume, lower-speed roadways, mini-roundabouts are a potential game changer in terms of enabling a cost-effective roundabout option.

So let's create a "virtuous circle" - by using the mini-roundabout at appropriate locations in our region.

  Courtesy Transportation Research Board - March 2017 webinar on mini-roundabouts

Courtesy Transportation Research Board - March 2017 webinar on mini-roundabouts

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