A38 Derby Junctions

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Haydn1971
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A38 Derby Junctions

Post by Haydn1971 » Wed Jul 18, 2012 06:11

To be fair Graham, it's not about the delay caused to off peak users, but the benefits to many more times peak users, which in this age is typically 6:30am till 7:30pm in most major built up areas.

Having designed many a junction, it can be difficult to get a signal controlled roundabout to work safely without signals - due to the restrictions of entry path curvature when approaches are widened to accommodate extra lanes for the signals over what was needed for traditional styles of roundabouts.

Whilst I accept some highway authorities just slap signals on the existing geometry, there may be other factors at play, such as throttling of a non-priority movement, casualty problems etc.

To that end, in my opinion, slapping signals on an existing roundabout geometry is generally a compromise in many aspects, but sometimes the only realistic option, but going part time has it's problems too, in that some users get confused by the deployment of both signals and give way lines, in that they set off on a green, then attempt to give way a few yards into the roundabout entry to the traffic on the circulatory that is already held on red. Read through is also an issue, where some users see the big give way signs, but miss the little red stop signal. The key is that not everyone drives the same and some drivers fall very short of a required standard, but as highway designers, we still have to consider their impact on all users.
Regards, Haydn

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Re: A38 Derby Junctions

Post by Yorkie » Wed Jul 18, 2012 12:58

Haydn1971 wrote: To that end, in my opinion, slapping signals on an existing roundabout geometry is generally a compromise in many aspects, but sometimes the only realistic option, but going part time has it's problems too, in that some users get confused by the deployment of both signals and give way lines, in that they set off on a green, then attempt to give way a few yards into the roundabout entry to the traffic on the circulatory that is already held on red.
Guilty as charged. I manage to do this every time I use the A38/A61 rbt north of Derby; following convention at the Green Light and then subsequently at the give way line. I'm aware that my action in doing this delays people and yet I blame it on bad design (like I do when roundabout lanes don't follow properly). I am entirely responsible for my action which I can't fully explain, though I wonder if it's not helped latterly by poor grass maintenance on the right hand side as you approach on the A38 from Derby going North reducing sightlines.

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Re: A38 Derby Junctions

Post by Graham » Wed Jul 18, 2012 13:56

Haydn1971 wrote:To be fair Graham, it's not about the delay caused to off peak users, but the benefits to many more times peak users, which in this age is typically 6:30am till 7:30pm in most major built up areas.
All fair points, Hardn, but your post does give the overall impression that it's only Peak Time that matters; users outside these times can go hang.
Haydn1971 wrote:The key is that not everyone drives the same and some drivers fall very short of a required standard, but as highway designers, we still have to consider their impact on all users.
Maybe if part-time signalised roundabouts were more common, people would get more used to them and these problems would disappear...

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Re: A38 Derby Junctions

Post by Graham » Wed Jul 18, 2012 14:05

swarkestonecauseway wrote:The irony is that your suggestion is in place on one of the Derby junctions, and we are all clammering for it to go
I did see this in one of your earlier posts, and I must say I found it odd. Any chance of explaining what the issue is?

I do know the Derby roundabouts intimately - I was brought up in Allestree and my old Dad still lives there.

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Re: A38 Derby Junctions

Post by SC2 » Wed Jul 18, 2012 19:33

Nice to see someone else from round here :)

Don't know the root cause of the issue to be honest- capacity of the junctions just too low?- but I know the symptoms of the issue if you like. Namely congestion and a high accident rate.

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A38 Derby Junctions

Post by Haydn1971 » Wed Jul 18, 2012 20:03

Graham wrote:All fair points, Hardn, but your post does give the overall impression that it's only Peak Time that matters; users outside these times can go hang.
When looking at the bigger picture, yes... If you can get 15-25% more traffic through a junction for 16hrs a day, then the small amount of delay for users through the night is a small weight to carry.
Graham wrote:Maybe if part-time signalised roundabouts were more common, people would get more used to them and these problems would disappear...
They used to be many part time junctions, they were converted for a reason !

Also, you may have missed my comment about differences in geometry requirements... Sadly many engineers don't understand entry path issues, which is why some signal controlled roundabouts are modified such that they wouldn't be safe if the traffic lights were turned off.
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A38 Derby Junctions

Post by Haydn1971 » Wed Jul 18, 2012 20:24

swarkestonecauseway wrote:Don't know the root cause of the issue to be honest- capacity of the junctions just too low?- but I know the symptoms of the issue if you like. Namely congestion and a high accident rate.
Quite simply that roundabouts have a lot of time wasted due to gap acceptance at the give way line.

The best way to get the most number of vehicles through a junction is to push them through as a platoon with minimal gaps between, once the gaps start increasing due to speed, capacity is being wasted, so that's when you get a red signal, giving the next platoon of traffic on a different approach a bite of the capacity available. You have to remember that saturation of a single lane is typically about 1800 vehicles per hour, although I've seen as high as 2100... But with higher speeds, that figure drops, as the gaps between vehicles naturally gets wider.

So in very simple terms, a crossroad with equal flows including the intergreen period between the two stages means that about 800 vehicles per hour can get across a stop line, add a second lane at the stop one, you might assume that 2x800 vehicles might cross the stop one, but once traffic gets a flow on, the second lane becomes of less value, as traffic has to return to the one lane at the other side of the junction. At risk of upsetting the resident signals guys, a good starting point for a full cycle of signal stages is 80 seconds, increasing the cycle time risks introducing waste from gapping, decrease risks introducing waste from intergreen times. But of course, it's much more complex than that and supports a whole industry of people like me ;-)
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Re: A38 Derby Junctions

Post by Graham » Wed Jul 18, 2012 23:13

Haydn1971 wrote:When looking at the bigger picture, yes... If you can get 15-25% more traffic through a junction for 16hrs a day, then the small amount of delay for users through the night is a small weight to carry.
As I said before, I agree with your points, but I think there is an even bigger picture. Ultimately, the authorities will get more respect from road users if they demonstrate that they are taking the needs of all users into account, not just the people who use the road most. And, more to the point, spend the limited budget wisely!

Here's an example of what I am getting at. My local motorway junction - M5 J9 - has recently been "upgraded" with traffic lights at both of the off-slips. This appears from the motorist's point of view to be a poor decision - this is not a particularly busy junction, and the congestion which you get at this junction during the morning and evening rush hours is caused by under-capacity signalised junctions on the A46 through Ashchurch - this leads to traffic backing up to the motorway roundabout. Predictably, the new traffic lights have done nothing to ease the congestion. As these signals have simply been added to an existing junction, there are no geometry issues, so why not turn the lights off outside of peak hours? That would ease the frustration of the motorist who uses the junction regularly, who has seen money wasted solving the wrong problem (the money spent on remodelling M5 J9 should instead have been used on the Ashchurch junctions, which is where the actual problem is).

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Re: A38 Derby Junctions

Post by Graham » Wed Jul 18, 2012 23:26

swarkestonecauseway wrote:Nice to see someone else from round here :)
I haven't lived in Derby since 1984, but I do remember the Allestree Link Road being built. One of the residents in the street I lived in (Carsington Crescent) started a petition to object to the building of the road, because the noise pollution would be unacceptable. Of course, the road is actually in a cutting, and so the noise pollution is non-existent. It was an early lesson to me that NIMBYs don't necessarily research their case properly, and since then I have always taken comments from NIMBYs with a pinch of salt.
swarkestonecauseway wrote:Don't know the root cause of the issue to be honest- capacity of the junctions just too low?- but I know the symptoms of the issue if you like. Namely congestion and a high accident rate.
I wonder whether it is linked to the comments that Yorkie made, which in turn must be down to the design of the junction. The part-time signalised roundabouts at Gloucester don't suffer from these problems.

Graham

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A38 Derby Junctions

Post by Haydn1971 » Wed Jul 18, 2012 23:27

I'm guessing the roundabout is HA and the other junction is not ?
Regards, Haydn

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Re: A38 Derby Junctions

Post by Chris Bertram » Thu Jul 19, 2012 09:43

Haydn1971 wrote:I'm guessing the roundabout is HA and the other junction is not ?
The A46 certainly used to be trunk, is it not now?
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Re: A38 Derby Junctions

Post by Jeni » Thu Jul 19, 2012 11:57

Chris Bertram wrote:
Haydn1971 wrote:I'm guessing the roundabout is HA and the other junction is not ?
The A46 certainly used to be trunk, is it not now?
I do believe the A46 is still trunk there.

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Re: A38 Derby Junctions

Post by wrinkly » Thu Jul 19, 2012 12:23

It appears from Google satellite view that there are signal-controlled side road junctions on the A46 at short, roughly equal distances both sides of M5 J9. I imagine the western one is non-trunk.

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Re: A38 Derby Junctions

Post by Yorkie » Thu Jul 19, 2012 12:55

Yorkie wrote:
Haydn1971 wrote: To that end, in my opinion, slapping signals on an existing roundabout geometry is generally a compromise in many aspects, but sometimes the only realistic option, but going part time has it's problems too, in that some users get confused by the deployment of both signals and give way lines, in that they set off on a green, then attempt to give way a few yards into the roundabout entry to the traffic on the circulatory that is already held on red.
Guilty as charged. I manage to do this every time I use the A38/A61 rbt north of Derby; following convention at the Green Light and then subsequently at the give way line. I'm aware that my action in doing this delays people and yet I blame it on bad design (like I do when roundabout lanes don't follow properly). I am entirely responsible for my action which I can't fully explain, though I wonder if it's not helped latterly by poor grass maintenance on the right hand side as you approach on the A38 from Derby going North reducing sightlines.
Probably sounds like I'm talking to myself here, but I think I worked the issue out last night whilst explaining this to a friend at a BBQ.

Signal controlled roundabouts have signals on them. You enter on green and then give way is unecessary. You then stop part way round as red signals dictate.

Signal controlled entry roundabouts do not have signals on them. You have a green to enter then you must give way.

I'm not sure what the terms for these different systems are, but is there a signage difference between the two for the average driver? In Sheffield we have several part time versions of the second type, the one north of Derby on the A38 is the first type (part time)

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Re: A38 Derby Junctions

Post by Chris Bertram » Thu Jul 19, 2012 12:56

wrinkly wrote:I imagine the western one is non-trunk.
The road into Tewkesbury is not even the A46, it's the non-primary A438. As was the A46 as far as Teddington Hands until about 20(?) years ago.
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Re: A38 Derby Junctions

Post by Graham » Sat Jul 21, 2012 21:48

wrinkly wrote:It appears from Google satellite view that there are signal-controlled side road junctions on the A46 at short, roughly equal distances both sides of M5 J9. I imagine the western one is non-trunk.
The western junction (on the A438) is not in itself a problem. You do get congestion here in the evening rush hour, but this is a knock-on effect of the congestion on the roundabout at M5 J9, which is caused by the junctions to the east of the junction, on the A46. Note that there are two signalised junctions - the first leads to an industrial estate, and the second provides access to Northway estate and Ashchurch railway station.

The main problem seems to be that an overly-generous amount of time is allocated to the minor roads at these two junctions, but it may be that some imaginative redesign could help matters.

Graham

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