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Author Topic: Unexpected traffic chaos near Melksham – what it costs, and how to deal with it  (Read 1957 times)
grahame
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« on: February 19, 2020, 09:06:14 pm »

An exercise in the arithmetic of traffic jam finances, and a lesson that a few more vehicles can cause a lot more congestion. 

Background ...

There are two road bridges across the river Avon in Melksham - the Town Bridge which feeds direct into the Town Centre (and it's the A3102 as well, the road east from Melksham) and the Bypass Bridge on the A350 "Western Bypass".  From 10th February, planned until 5th May, the Town Bridge is planned to be open to northbound traffic only, while pedestrian railings are fitted between the pavement and roadway.  The effect of this is that traffic arriving into the town from the north - or the North West (A365) or Bradford-on-Avon (B3107) all has to go around the Farmer's Roundabout, go round the bypass (which is just one lane each way). If it wants the Town Centre, many residential areas, or the A3102, traffic has to loop back.   Let me add a map to help you


Red pin - Town Bridge; Green pin - Bypass Bridge

All very well and good - except that the traffic around the bypass, southbound, in the evening peak hour is already close to saturation.  And once it passes saturation, it backs up in the form of traffic jams.   It only takes a little to tip the balance.

Added to which ... BaNES closed the Cleveland Bridge in the City to HGVs on 5th February, and the recommended diversion from the Motorway (A46 into Bath) to the A36 (on from Bath to Southampton) is - you guessed it - along the A350 past Melksham.  Even if the number of vehicles is small, with the bypass bridge already close to capacity, this could push it over from "coping" to "queueing".

Whilst some disruption was expected, the scale of congestion appears to have taken the Highways Department by surprise.

My analysis of the problem and its cost

From The BBC - cost per hour that a driver is stuck in a jam - just over £30. This is, I believe, the figure used when evaluating road schemes.

Number of cars and vans travelling south along the A350 Western Way in Melksham over a the five minute period from 17:30:00 to 17:35:00 on 19th February 2020 - 130 (also 3 HGVs, no cycles, no motorcycles).  I have no way of knowing if those HGVs were ones that would normally use the A350, whether they would normally have used the Town Bridge to reach the Calne Road, or whether they would normally use the A46 / A36 link in Bath.

Traffic jam on A350 from Chippenham (last night, Lisa's report) - she took an hour and 20 minutes for a journey than would normally take just under 20 minutes, so that's a 60 minute delay

130 cars in 5 minutes = about 1550 cars per hour. Now - not everyone's coming down the A350 - there is also joining traffic from Bath on the A365, blocked back (I understand) to Shaw at times, and joining traffic from ASDA supermarket, and from Aldi, McDonald's and residential streets such as Avon Road, Granville Road and Scotland Road, and the B3107. Here I'm making an educated guess - that's an "average" motorist at the peak time is delayed by 45 minutes. Happy to talk that guess through - I suspect it's on the low side.   So in an hour the cost to motorists, using the 2 year old BBC price, my optimistic estimate of 45 minutes and the £30 figure which is rounded down from the examples in the BBC report is £46,500.00 ... incredible what the cost of the jam is costing in each evening peak hour, using the government's own cost figure, isn't it?

What to do about it?

As from this Sunday, Wiltshire Council are probably reversing the flow over the Town Bridge - making is Southbound only. But is that simply going to turn the problem from an evening southbound one into a morning northbound one?  They don't think the problem will be as severe; not sure I understand their logic, but they are (after all) the experts.  It seems to my simplistic mind that having the town bridge outbound in the morning peak and inbound in the evening peak would make much more sense, so I asked if that would be possible, and a couple of other related questions too.

Answers to questions

I'll quote exactly

1. Is there a problem in the MORNING peak, or just the evening?  If just the evening, that indicates an asymmetric flow and the problem would be largely alleviated by having the Town Bridge northbound in the morning and southbound in the afternoon.

Quote
We are screen dumping the traffic congestions plans from Google frequently, there doesn’t appear to be any back up in the mornings heading north but do bear in mind north bound flows are at the moment unaltered as traffic can still cross the bridge in that direction. This will of course change after Sunday when northbound traffic is prevented from crossing the bridge. There are however more opportunities to turn off seek alternative routes when heading north.

2. As well as taking traffic from the Town Bridge, the bypass bridge is taking HGVs that normally use the A46/A36 Cleveland Bridge link in Bath.   Would it not have been better not to do both lots of roadworks at the same time - BaNES and Wiltshire agreeing to do them one after the other?

Quote
Data taken from the traffic counters on Farmers roundabout show the additional HGV traffic is negligible, the main tailbacks on the A350 have occurred after 5.00pm when HGV traffic tends to be less. The Cleveland Bridge restriction is likely to be in place for some time, once B&NES are granted Listed Building Consent they then need to bid for funding and then they can go out to tender and procure the works.

3. When the Bypass Bridge was also closed by the emergency services the other afternoon, the police advised motorists to “use an alternative route”.  What alternative route(s) are recommended to avoid the  bypass bridge at such times, and indeed while the road is merely congested at present?

Quote
Closure of the bypass last week was instigated by the police following reports of a person in distress in the water. We could not have foreseen this happening. Whilst the motorways have agreed alternative routes set up we don’t do the same for locally maintained roads. A planned closure would have an alternative route signed, we don’t have the resource to sign emergency closures as happened last week.

4. Why was this not foreseen - sounds like it was planned on “hope”?

Quote
Amy roadworks will inevitably cause disruption, we do what we can to keep this to a minimum. When works were undertaken on the bridge several years ago two way lights were used and this brought intolerable congestion to the town centre. In planning these works we liaised with the Town Council and the local Chamber of Commerce to try to work out a better solution. The use of the one way system was deemed to be ‘the best of a bad lot’. Closing the road to southbound traffic has had a greater than expected knock on effect on Farmers Roundabout, we accept this is causing disruption hence the changing the flow to see if this makes a difference.

Comment

I am delighted to have the answers from Wiltshire Council's Highways team.  All the answers are excellent and some addressed the question I asked  Cheesy.

1. I accept that the metrics may be different in the morning peak, and that traffic can turn off closer it to "rat run" via Melksham Forest, Woodrow and Lacock or even via Sandy Lane if headed for the centre of Chippenham. I hope that makes a difference; I'm neither convinced, nor am I enough of an expert. My question about "could you reverse the flow to have the bridge open in the busiest direction in each peak" was not answered, either with a reason why not, or an explanation of why it would not work.

My intermediary suggested to the council that there might be a "legal issue" switching daily, and I guessed it might relate to problems having someone switch signs each time, but neither of those explanations was offered.

2. Yes, the extra traffic from Bath is very little.  Not sure if it's "negligible" though in a system that's so very close to the tipping point.  The fact that there are more "heavies" earlier in the day (when there are far less cars) is interesting, but not relevant to the maths of the problem period. It's not those earlier times that are the problem - it's the peak.

3. Yes, the closure of the remaining bridge by the police was very unfortunate. Question remains - if the instruction from the authorities is "use an alternative route", what route?   Doesn't need signing - just post it on Facebook, have BBC Wiltshire tell people which way to go - thin traffic out that way.  Didn't hear any specific route advise, and indeed WC says they don't offer it ... while the police have their hands more than full.  Ignoring the one event day, specific advise on how to avoid the bypass bridge would be much appreciated!

4. Again, I agree - alternate working with traffic lights over the town bridge would gridlock the town. But why are you telling me this?   I didn't ask.  What I did ask about was northbound in the morning and southbound in the evening, and the silence in answering that question is deafening.

Multiply up the delay costs for an hour at present (£46,000) by 12 weeks of weekdays (60 days) and you get a very high price indeed for the motorist.  Delighted that the contractors are being asked to speed up the work - that will help.  Fingers crossed that switching the direction this weekend will lessen the jams.  But my question remains - why on earth not switch each midday and each midnight?  There is little problem as I understand it southbound in the morning, so the peaks flows are asymmetric enough such that this could save the motorists - using Department for Transport figures - literally millions!


The closed bridge


The closed bridge - the third day of works


Not only cars with one person, but buses with dozens are held up in queues


The A3102 approaching the bridge looks as quiet as a play street ...
... with all traffic arriving in the Town Centre up King Street.
« Last Edit: February 20, 2020, 05:23:20 am by grahame » Logged

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grahame
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« Reply #1 on: February 20, 2020, 10:34:59 am »

Original post on Facebook on this topic - ((here))



Follow up including links back to here, rail alternatives, link back - ((here))
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Bmblbzzz
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« Reply #2 on: February 20, 2020, 10:47:59 am »

No maths from me either, but the proportion of HGVs to cars must matter. Large vehicles not only occupy more road space, they take longer for each vehicle to clear a junction. Similarly, converting a few cars to motorcycles can free up much more space, as the motorbikes are not only smaller but tend to filter through jams rather than add to them. Buses must play a complicated role in this, each bus taking a couple of dozen cars off the road but again taking longer to clear a junction. And vans? Don't know...
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grahame
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« Reply #3 on: February 20, 2020, 01:03:28 pm »

No maths from me either, but the proportion of HGVs to cars must matter. Large vehicles not only occupy more road space, they take longer for each vehicle to clear a junction. Similarly, converting a few cars to motorcycles can free up much more space, as the motorbikes are not only smaller but tend to filter through jams rather than add to them. Buses must play a complicated role in this, each bus taking a couple of dozen cars off the road but again taking longer to clear a junction. And vans? Don't know...

I was looking for no more than a rough flow figure last night ... noted minute by minute.   In each of the two minutes where no HGVs can through, there were 31 cars / vans.  In the minutes that HGVs passed, those numbers dropped to 27, 19 and 22 cars.  Not enough data to more than hint at an idea that lorries take up more "paths" - natural as their slower accelleration from the pinch point at the exit of the Farmer's Roundabout will hold back cars slotting in behind them until they're up to speed.

Although it's the town bridge one way system / closure that's causing the bypass to be oversubscribed at present, the building of more homes off Pathfinder Way, Snowberry Lane and Eastern Way that's underway - perhaps 1500 new homes - will oversubscribe it once again soon after the Town Bridge is re-opened.  And the continued expansion of Trowbridge will add further traffic.   There are definite plans now - and it looks like funding - for a bypass to bypass the bypass.
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