M4 J19 - Just why?

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DB617
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M4 J19 - Just why?

Post by DB617 »

This has been bothering me on my new morning commute Bath-wards. Ever since I first started driving, I've thought this junction weird. Why is there an unmarked increase from two lanes to three above the interchange, followed swiftly by a busy inter-motorway slip road joining from the left? Since I started using the slip daily, it's become more baffling to me. It would be far safer for the large volumes of traffic joining from Bristol to have access to a lane gain, increasing the M4 back to 3 lanes at the same time. Does anyone know what the thinking was behind this lane-drop-without-lane-gain? Is there any feasible traffic engineering reason for this, or is it just a botch?
Bomag
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Re: M4 J19 - Just why?

Post by Bomag »

DB617 wrote: Sat Aug 06, 2022 14:23 This has been bothering me on my new morning commute Bath-wards. Ever since I first started driving, I've thought this junction weird. Why is there an unmarked increase from two lanes to three above the interchange, followed swiftly by a busy inter-motorway slip road joining from the left? Since I started using the slip daily, it's become more baffling to me. It would be far safer for the large volumes of traffic joining from Bristol to have access to a lane gain, increasing the M4 back to 3 lanes at the same time. Does anyone know what the thinking was behind this lane-drop-without-lane-gain? Is there any feasible traffic engineering reason for this, or is it just a botch?
If designed properly then the lane drop and merge would have been designed to TD22 (CD122) and the relevant capacities would have been aligned to traffic flows. If the intra junction flows are too high then there will be flow breakdown and joining traffic will be faster than the mainline traffic which would encourage traffic in LBS2 (lane 1 intra junction) to cut across to the joining lane. This is both unsafe and stuffs capacity. By creating capacity intra junction the increase in capacity will lead to more consistent speeds and headways on the mainline which should create merge opportunities in lane 1.

On the other had it could be just the SMG dogmatically sticking to the scheme end point (i.e. intra-juction) and refusing to touch the merge layout.
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KeithW
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Re: M4 J19 - Just why?

Post by KeithW »

DB617 wrote: Sat Aug 06, 2022 14:23 This has been bothering me on my new morning commute Bath-wards. Ever since I first started driving, I've thought this junction weird. Why is there an unmarked increase from two lanes to three above the interchange, followed swiftly by a busy inter-motorway slip road joining from the left? Since I started using the slip daily, it's become more baffling to me. It would be far safer for the large volumes of traffic joining from Bristol to have access to a lane gain, increasing the M4 back to 3 lanes at the same time. Does anyone know what the thinking was behind this lane-drop-without-lane-gain? Is there any feasible traffic engineering reason for this, or is it just a botch?
Well this is a road that opened in 1966 and I suspect that back then it was rather quieter than it is today, it certainly was in the 1970's when I first drove it. There are lots of older junctions with these issues , see A1(M) at Scotch Corner.
https://www.google.co.uk/maps/@54.43650 ... 8192?hl=en

Or M2
https://www.google.co.uk/maps/@51.33309 ... 8192?hl=en

M11 at J8
https://www.google.co.uk/maps/@51.86600 ... 8192?hl=en
DB617
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Re: M4 J19 - Just why?

Post by DB617 »

Bomag wrote: Sat Aug 06, 2022 16:13
DB617 wrote: Sat Aug 06, 2022 14:23 This has been bothering me on my new morning commute Bath-wards. Ever since I first started driving, I've thought this junction weird. Why is there an unmarked increase from two lanes to three above the interchange, followed swiftly by a busy inter-motorway slip road joining from the left? Since I started using the slip daily, it's become more baffling to me. It would be far safer for the large volumes of traffic joining from Bristol to have access to a lane gain, increasing the M4 back to 3 lanes at the same time. Does anyone know what the thinking was behind this lane-drop-without-lane-gain? Is there any feasible traffic engineering reason for this, or is it just a botch?
If designed properly then the lane drop and merge would have been designed to TD22 (CD122) and the relevant capacities would have been aligned to traffic flows. If the intra junction flows are too high then there will be flow breakdown and joining traffic will be faster than the mainline traffic which would encourage traffic in LBS2 (lane 1 intra junction) to cut across to the joining lane. This is both unsafe and stuffs capacity. By creating capacity intra junction the increase in capacity will lead to more consistent speeds and headways on the mainline which should create merge opportunities in lane 1.

On the other had it could be just the SMG dogmatically sticking to the scheme end point (i.e. intra-juction) and refusing to touch the merge layout.
There's something in that. Ending the smart motorway before J19 on the eastbound side was one hell of a missed opportunity. Two lanes of solid traffic come in from the M32 and merge with three lanes, all down into three, 500 yards after the end of a smart motorway... a mile or so of extra HSR/ALR might have given the opportunity for a smoother merge without the bodge.
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SouthWest Philip
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Re: M4 J19 - Just why?

Post by SouthWest Philip »

Another problem with the double lane drop is it exacerbates delays for seasonal flows (i.e. holiday traffic) at times when perhaps the traffic to/from the M32 and Bristol might be lower. To my mind, reinstating the 3-lanes throughout for the M4 eastbound with an extended merge from the M32 might be a better compromise. Even with the through lane reinstated it would still be possible to use lanes 1 and 2 for the M32.
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Bryn666
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Re: M4 J19 - Just why?

Post by Bryn666 »

Once again our National Highway overlords have just got very little clue how actual traffic operates beyond what their spreadsheets and modelling tell them. I doubt they've ever observed real world behaviour in situations like this - traffic never darts across into an "empty" lane 1 when there's traffic merging from the left at higher speeds on the M60, which is all the bloody time approaching J13 and which is why there's always a 5 mile tailback at peak times.

In fact maintaining three through lanes at M60 J12 when there's so much entering traffic is the worst thing they could've done but hey ho.
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DB617
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Re: M4 J19 - Just why?

Post by DB617 »

Bryn666 wrote: Sat Aug 06, 2022 19:15 Once again our National Highway overlords have just got very little clue how actual traffic operates beyond what their spreadsheets and modelling tell them. I doubt they've ever observed real world behaviour in situations like this - traffic never darts across into an "empty" lane 1 when there's traffic merging from the left at higher speeds on the M60, which is all the bloody time approaching J13 and which is why there's always a 5 mile tailback at peak times.

In fact maintaining three through lanes at M60 J12 when there's so much entering traffic is the worst thing they could've done but hey ho.
My thoughts exactly. In fact, at lane gains it's often quite the opposite. Most HGV drivers aside, it's not uncommon to see drivers continue in lane 2 for several miles. Welsh drivers are awfully prone to it at the J21 lane gain and are often seen doing the entire PW bridge in lane 2.
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