Roundabout Frequency

[TerryScan]

The recent activity in the thread on Roundabout Truck Aprons (

http://www.eng-tips.com/viewthread.cfm?qid=216605&page=1)

prompted me to ask this question (but I didn't want to confuse the previous thread topic).

I am in the US and our state DOT is quite enthusiastic about roundabouts. Questions I have for the Europlean folks who are used to these animals are:
1. Is there any relevance to often are roundabouts usually placed? (1 every X intersections).
2. or can it be common to have roundabouts in sequence for a number of intersections in a row?
3. ie. would it be normal to find roundabouts at every intersection in a rural area where intersections are not close together?
4. would it be normal/functional even if the intersections are close together in an more urban setting? (how close it too close?)

 

[francesca]

I can't think of any reason to space roundabouts out. In my experience, the vast majority of intersections on local suburban and rural roads in the UK are roundabouts. My favorite roundabout intersection has always been Hatton Cross, even if it is more strictly a gyratory.

 

[Debaser]

TerryScan

As Francesca says, for us in the UK there's no problem having roundabouts in sequence. The use of roundabouts is usually dictated by the number of arms approaching the junction, the volume of traffic (and relative volumes of turning movements) and the space that's available. (And, of course, the personal preferences of whoevers judging the planning application at the Local Authority).

Our main concerns would be;
Urban - Mixing roundabouts with signalised junctions means UTC systems can be difficult to introduce (roundabouts are just glorified give-way/yield junctions),
Rural - Continuity of provision. If you've got a number of one type of junction, throwing in a different type as a one-off, when there may be no apparent need could cause confusion.

As far as how close can you site them, ultimately that's dependant on the lenght of queueing you might expect. Blocking back is not good, although on our network we get it all the time in urban situations. If you're looking at new build (and in a 'land rich' country) then I'd just do some modelling with your preferred software to determine potential queue lengths in some future design year and go with that.

BTW, attached is a pic of Hatton Cross showing what we do get up to. English eccentricity at its finest!

 

[Debaser]

Further to the attached photo above. THAT roundabout is in Swindon, THIS roundabout is Hatton Cross.

 

[kellig]

What is the safety record like on that crazy Hatton Cross Roundabout?

 

[TrafficDesigner]

Those are great photo samples. Does the queue for the Swindon roundabouts ever back up?

 

[Debaser]

These two are probably the most 'way out' examples of junctions in the UK (although Swindon has past 'form' for this kind of thing), see;

http://www.swindonweb.com/index.asp?m=8&s=115&ss=289&t=THE+MAGIC+ROUNDABOUT

and,

http://www.strum.co.uk/wessex/brunpic.htm

I hope Francesca will correct me, but I assume Hatton Cross was a large convential five-arm roundabout, subsequently retrofitted with five mini-roundabouts for some reason.

Unfortunately I don't know the safety records of either roundabout, however I'd imagine they're pretty good. I would think traffic speeds will be relatively slow and there will be a lot of eye contact between drivers entering and circulating. Since adopting the 'give-way to circulating vehicles' approach in the '60s roundabouts have been seen as the safest form of junction control in the UK.

In terms of backing-up, looking at the wider area I assume the east-west/west-east movements would be the worst, since this route forms a direct link to the motorway south of the town. However, you basically get a choice of two routes through the junction, i.e.
East to West - turn left (south) at the first roundabout, right at the second and straight-ahead at the third (to exit), or,
Straight-ahead at the first (to get to the northernmost rbt), left at the second and right at the third (to exit).
This route choice may lead to an equalisation of traffic movements through the intersection?
In any event not something I'd be volunteering to model.

(BTW to give you an idea of scale, the Swindon junction is approximately 120-125 yards east-west and north-south, to the centres of the islands, and Hatton Cross has an ICD of approximately 85 yards).

 

[Debaser]

For general design information 'the UK way' the link takes you to our national design standards , the Design Manual for Roads and Bridges (DMRB), in particular Volume 6 (of 15), Section 2, Junction Design.

http://www.standardsforhighways.co.uk/dmrb/vol6/section2.htm

Look for TD 16/07 'Geometric Design of Roundabouts', though there may be others of interest.