L5 Hamburger Expert Needed A408 Heathrow

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L5 Hamburger Expert Needed A408 Heathrow

Postby AndyPot2 » Wed Jun 04, 2014 5:41 pm

L5 Hamburger roundabout expert needed to tinker with this roundabout in West Drayton just north of Heathrow? It generates the odd UR and there is one there at present. I would ask for it to be downlocked but think it would be better for a L5 who has experience with these roundabouts to do it directly.

I don't think there are any major problems but some of the things I've picked up are:

1. The slip road segment I have linked to above is connected to the off slip from the north side of the A408! Therefore Waze actually directs you exit the roundabout and then drive 6.5 feet the wrong way up a slip road and then turn right :roll: . Surely it should be connected directly to the roundabout.

2. The geometry on these two segments is not correct. They cause a turn left instruction after exiting the roundabout.

Comments / Discussion points.

I have always considered this as a roundabout with a cut through and never had a problem with it but probably don't listen to the Waze instructions as I know it well, however I acknowledge that others may view it differently and possibly it may work better if drawn as a one way road rather than a roundabout.

To support the roundabout view note that if approaching the roundabout from the SW along Holloway Lane or from the E along Shepiston Lane there are roundabout type signs showing the layout of the junction.

However to support the opposing view

1. Note that signs in the middle of the roundabout consist of a white arrow in a blue circle with a sign underneath saying Dual Carriageway (never noticed that on a roundabout).
2. It is possible (in fact you are directed) to turn right from the roundabout into the centre (ie onto the meat of the hamburger), I don't think we can map a hamburger roundabout to work like that!
3. If approaching from the NW down Cherry Lane there is no sign of a roundabout sign and once on "the roundabout" the road sign treats it more like a cross roads with a turn left for Uxbridge, right for M4 and straight on for everything else.


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Re: L5 Hamburger Expert Needed A408 Heathrow

Postby Timbones » Thu Jun 05, 2014 1:17 am

I'm familiar with this roundabout, and have discussed it's construction before. Not sure about that east-side exit though. I'll have a proper look at it when I get the chance... zzz
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Re: L5 Hamburger Expert Needed A408 Heathrow

Postby eberwolf » Wed Jun 11, 2014 6:00 am

Sorry it's taken a while to add my thoughts to this, but I've not been well.

I lived in this area for nearly 4 years, so I'm reasonably familiar with this intersection too. Personally, I've never considered this to be a roundabout (hamburger or otherwise). I suspect however that the argument over whether or not this is technically a roundabout will never be completely resolved :|

Maybe a better question is 'Should this be mapped as a roundabout?'

My main focus when 'fixing' junctions to date has been to ensure Waze provides 'useful' TTS directions - not just correct routing. I have noticed that there are many junctions where it seems a lack of care has been taken over the geometry resulting in confusing spoken directions (an observation not a criticism btw :D )

With this in mind, the main issue becomes whether or not it makes sense to provide 'take nth exit' style instructions. Whilst there are junction layout signs that could be construed as roundabout signs on a couple of the side roads, it is not easy to identify the number of 'exits' from these - and when you reach the junction in moderate traffic, it is not easy to count the exits as you pass them. Especially considering
AndyPot2 wrote:2. It is possible (in fact you are directed) to turn right from the roundabout into the centre (ie onto the meat of the hamburger), I don't think we can map a hamburger roundabout to work like that!

Traffic flow is primarily lane based. If approaching from any of the other directions, there is no indication you are entering a roundabout (as Andy points out). In fact, many of the more 'local routes' virtually bypass the central round section altogether. As an example, A408(S) to Shepiston Ln is signposted (with matching road markings) as a left turn.

From this perspective, I would posit that mapping as a roundabout does not provide the best TTS directions - although I appreciate that fixing this is not a trivial task.

Observation regarding road naming
I would suggest (based on road signage and OS maps) the the A408(S) FW segment, should be labelled as the M4. Currently, directions for a route between Shepiston Ln & the M4 roundabout tell you to turn left on A408(S) - which contradicts road markings and signage. This would also apply to the A408(N) FW segment (even though in this instance it could provide less clear directions :roll: )

Note: This intersection now has 3 URs associated with it

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