Roundabouts

This page is under construction and won't be finished for a while.

Overview

Roundabouts can be hugely beneficial to traffic flow and safety at some junctions - if you use them properly.

Note the two key factors mentioned above:

• traffic flow
• safety

Both of these factors can be examined through the lens of conflict points:

• Conflict points are accident-prone
• To avoid accidents, traffic has to be more cautious
• Cautious traffic is generally slower (which can be good in some situations)
• But traffic jams lead to frustration, which often leads to accidents

Roundabouts can help reduce the number of conflict points, and simplify those that remain.

Mini roundabouts

These are the smallest and most basic roundabouts, usually found at small T- and Y-junctions. They are usually just a painted dot in the middle of a junction, with some arrows to clarify direction of traffic, like this:

todo: diagram

Side note: A benefit here, on small junctions, is that large vehicles can just drive over the painted dot; they don't have to turn a tight circle. If there's lots of them, however, it will cause significant disruption to the traffic flow.

How does this help? Let's look at the conflict point diagrams:

todo: diagrams

We've gone from 9 conflicts to 6, a 33% reduction. Here's a table of before vs. after:

Conflict	Before	After
Merging	3	3
Crossing	3	0
Diverging	3	3

By forcing traffic to drive in a circular path around the junction (clockwise for LHT/RHD, anticlockwise for RHT/LHD), we've completely eradicated the crossing conflict points.

But what about the other conflict points, have we improved those at all?

Because drivers must yield before entering the roundabout, we've transitioned from "moving merges" to "waiting merges". That's a big increase in safety, because now drivers have more time to think and appraise the situation. Likewise, we've simplified the diverging conflicts; the driver is really only concerned about what's ahead of them because we removed those crossing conflicts. We've also swapped the order of the remaining conflicts; before, drivers would diverge then merge, but with the roundabout they now merge, then diverge.

So, not only did we reduce the number of conflict points, but we simplified those that remain.

Normal roundabouts

So far, we've just been thinking of cars going through a simple junction. But, what happens if we've got lots of large vehicles? Or cyclists? Or pedestrians? Or more roads? In those cases, we need a slightly larger and more elaborate roundabout.

The basic principles are just the same; we're just applying them to more stuff.