How Do You Exit A Roundabout. Incident Two.

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sirmy
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Re: How Do You Exit A Roundabout. Incident Two.

Postby sirmy » 21 Jul 2018, 10:44pm

foxyrider wrote:As far as I can see you took the route I would have which is the route the 'designers' intend you to use making this manoeuvre.

Using the LH lane is both incorrect and dangerous - the TI near me is marked on one approach as LH+ S and the adjacent exit has a collision almost every day - the other approaches are marked LH only and don't have the issue.


Totally agree but there seems to be an increasing number of drivers who think that being in the left lane is correct when doing a manoeuvre like that, and of course only indicating just before the exit they want. I had a near miss a few weeks ago when I stupidly assumed a driver in the left lane, with no indicators visible, was taking the first exit across and not the second, stupidly I pulled out and only a heavy foot on the accelerator saved the day, good job I wasn't on the bike! I'm back to assuming every other driver is an idiot, and sadly not being proved wrong.

wjhall
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Re: How Do You Exit A Roundabout. Incident Two.

Postby wjhall » 23 Jul 2018, 3:32pm

Using the LH lane is both incorrect and dangerous - the TI near me is marked on one approach as LH+ S and the adjacent exit has a collision almost every day - the other approaches are marked LH only and don't have the issue.


The OP appears to have taken the route intended by the designers, and also the standard route for turning right, apart perhaps from merging early. The traffic jam also appears to be taking the design route but not the usual route, which would have been left lane for straight on. The problem is, of course, the merge, a standard design feature on roundabouts and at traffic lights, where designers believe that two lanes of traffic will merge smoothly into one, by some magic. (Presumably less magic than the twelve into four point above.) In this roundabout this should in principal be dealt with by traffic on the straight on route giving way to traffic on the turn right route as it enters, but this approach has probably been mitigated against by the relative size of the flows, and the lane separation hatched lines actually in the roundabout. There should probably be yellow box hatchings in the roundabout.

The other roundabout described in the post quoted above shows another problem. There are standard approaches, the left lane is usually for Straight on, you cannot be too surprised if people who do not know the junction assume otherwise and have to 'correct' to the marked approach rather late.

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mjr
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Re: How Do You Exit A Roundabout. Incident Two.

Postby mjr » 24 Jul 2018, 1:49pm

wjhall wrote:The problem is, of course, the merge, a standard design feature on roundabouts and at traffic lights, where designers believe that two lanes of traffic will merge smoothly into one, by some magic.

I don't think they believe in magic. I believe they do it because it makes the approach queues half the length (which looks good on the stats) and moves half the vehicles away from the roadside pollution monitors - and any failed-to-merge crashes can be blamed on substandard driving.
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wjhall
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Re: How Do You Exit A Roundabout. Incident Two.

Postby wjhall » 24 Jul 2018, 2:33pm

mjr wrote:
wjhall wrote:...where designers believe that two lanes of traffic will merge ..., by some magic.

I don't think they believe in magic. I believe they do it because it makes the approach queues half the length (which looks good on the stats) and moves half the vehicles away from the roadside pollution monitors - and any failed-to-merge crashes can be blamed on substandard driving.


Could be so, although I think the real issue is that roundabouts, like gyratories, only work with fairly low traffic flows, as in the DoT publicity photos from the '50s, with about five cars in sight. Once there are six cars the junctions no longer flow smoothly. The inevitable, necessary, solution is traffic lights, which would probably have allowed much simpler junction designs if they had been installed in the first place, see for example the top of Blackboy Hill, and Cumberland Basin in Bristol, both of which now have traffic lights, and in the last case, some of the junctions designed to connect everywhere to everywhere have been closed off, so simpler light controlled crossroads would probably have done the job in the first place.

More fundamentally these multi-lane structures are inimical to cycling, although for motor vehicles letting three out every time there is no conflicting traffic from the right probably does speed things up. My yellow box suggestion also helps motor vehicle flows, but again, is not necessarily good for cycling. You may think you can slip across the box from the right lane and merge beyond it, but you risk conflict with someone accelerating into the apparently empty box.

Roundabouts on main roads do often now seem to be built with traffic lights in them; I am not clear whether there is an advantage over a simple light controlled crossroads.

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Re: How Do You Exit A Roundabout. Incident Two.

Postby Vorpal » 24 Jul 2018, 2:43pm

It is important to keep in mind that the designers of these things are not particularly concerned with making it easier for the users. Their primary goal is maximising throughput, becuase that's what their designs are judged upon.

While there is a realtionship between maximising throughput and simplifying things for motorists, that is sometimes to the detriment of non-motorised users.

Also, traffic light serve the same principle, although they are generally aimed at letting traffic from a secondary flow join a primary flow (one that might not otherwise let them in, i.e. from a minor road onto a trunk road). Traffic lights are only included in the design when the traffic from the secondary is significant enough to build up without the traffic lights. Even then, they are timed to minimise inconvenience in the primary flow without excessive build-up on the secondary.
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