Stevie D wrote: ↑Mon Apr 15, 2019 10:31
What is the capacity difference of a roundabout with 3-lane approach and circulation and 2-lane exit, compared to a signalised junction with similar land-take? Genuine question, I would be interested to know the maximum peak-flow throughput, typical peak-time delays and typical off-peak delays for different models.
I can't provide any figures, but Jackal's US-sourced answer is probably near enough for roundabout capacity. The qualitative answer to your question though, as you've phrased it relative to land-take, is that a well-designed and properly marked 4-way signalised crossroad has more capacity than a 4-arm give-way roundabout. It simply doesn't have that big empty circle in the middle. The patch of asphalt within a well-designed cross-roads is constantly and densely full of moving vehicles. That patch, at an ex-roundabout site, is quite large.
A
fully signalised roundabout's figure will lie in-between. And let's not get into hamburgers!
When increasing demand is getting close to roundabout capacity, there will come a crossover figure at which the benefit flips over. Within a band of figures near there, the choice between the two options may be arbitrary. It becomes a psychological issue. Roundabout: long, long queues and delays under peak flow, delay-free off-peak. Cross-roads: higher and more predictable flow under load, irritating short delays at nearly every encounter, even when not busy. Tolerance for these different delay regimes will vary from driver to driver, and from society to society (country to country). And there will be a cultural and historical bias involved in any decision.
I would like the UK to reconsider part-time roundabout signalisation. I know we've discussed it many times. Although there's supposedly an issue about the approach geometry being different for the two cases, I don't really understand that in detail. What's the problem? Safety?
Also, an alternative is approach metering, to break dominant flows, create gaps, and thereby alleviate entry hesitancy. This has been tried at a number of Melbourne locations over many years, and has seemed effective to me, but only a couple of sites are still active. I don't know why. Has it also been tried in the UK?