Junction capacity

The cycling schemes in Bedford and Southampton – the ‘Turbo’ roundabout, and the Itchen Bridge junction, respectively – have been hitting the headlines recently. A post by SmallTown2K (who has been taking a thorough look at the Southampton scheme) goes some way towards explaining why what has ended up on the ground is so compromised –

In traffic engineering parlance, the junction does not operate satisfactorily in the AM peak. What this means is the junction is over capacity. I have no baseline Arcady (roundabout modelling software) for the roundabout to compare to, but the signals are likely lower in capacity and this indicates liable to cause congestion.

Obviously, traffic will readjust and vehicle congestion isn’t the be all and end all, except, in Southampton, almost everyone drives and angry drivers don’t re-elect people. Further, and perhaps less dramatically, as a highway authority, Southampton CC is bound to a Network Management Duty which means they must secure the “expeditious movement of traffic”, albeit that traffic is defined as all road users. In that vein it should be noted for non-locals that the Itchen Bridge is a key bus corridor and congestion over the bridge would affect all these routes and the large number of people thereupon (which offhand I would guesstimate outnumber cyclists in the order of 10:1).

It is this intersection of ‘keeping the traffic moving’ (conceived in terms of motor traffic) and political unwillingness to do anything that might disrupt ‘traffic’ (again, motor traffic) that has seen the removal of the ASLs from the original plans, the extra length of stacking lanes, and so on. The quality of the junction was sacrificed.

There’s a similar story behind the Bedford ‘Turbo’ roundabout. The council simply didn’t want to do anything that might have reduced the volume of motor traffic on the roundabout, resulting in the bodge that is finally going to see the light of day, with cycling effectively pushed onto shared use pavements, with a roundabout design that has the stated intention (whether it will succeed or not is another matter) of increasing motor traffic capacity.

The problem is that cycling is, as always, seen as something ‘extra’ to be accommodated around existing motor traffic, rather than a way of reducing congestion on the network as a whole. In a post yesterday Herbert Tiemens, of the Dutch Cycling Embassy, commented that

congestion easily evaporates with only a low percentage changing cars for bicycles

But we don’t seem to appreciate this in the UK – perhaps because we can’t get our heads around the fact that ‘ordinary’ people could actually switch from their cars to cycling, for short trips, if the conditions were more acceptable.

The truth is that designing junctions properly for cycling hugely increase the capacity of these junctions in terms of the movement of people, even if capacity for motor traffic is reduced. 

I dug out an old video of mine, shot in Groningen in 2011, just to demonstrate how efficient junctions can actually be.

This is the north-west corner of Vismarkt, right in the centre of the city, at about 5:30pm.

The video is only 3 minutes long, but I managed to count around 350 people passing through this junction in just that time – almost certainly an underestimate, because the video doesn’t capture people crossing on the arm to the right. This is a rate of 2 people every second, which amounts to at least 7000 movements per hour. It’s hard to say how many people might pass through here over the course of a day, but quite obviously the junction could handle a huge amount of people in a 24 hour period. All this in a small space, with no need for signalisation, or delay. And very little danger!

By comparison, busy junctions like the Bedford ‘Turbo’ roundabout currently handle 25,000 vehicles per day, as does the roundabout at the northern end of Lambeth Bridge – in a much bigger area, with much more delay, and with much greater danger. This junction in Groningen is much, much more efficient at moving people about.

I’m not suggesting that the motor traffic on these roundabouts can, or even should, disappear. The broader point is that shifting people out of their cars and onto bikes would serve to reduce congestion, not increase it – even if that means taking junction capacity away from motoring. But it has to be done properly, so that cycling is a genuine, attractive alternative.

This entry was posted in Infrastructure, Smoothing traffic flow, The Netherlands. Bookmark the permalink.

12 Responses to Junction capacity

  1. Mark Hewitt says:

    “congestion easily evaporates with only a low percentage changing cars for bicycles”

    This. Witness how different traffic is when a small number of people take holidays to coincide with school holidays.

  2. We’ve just made a “freedom of information” request to find out how much East Cambs District Council have in S106 (developer money designed to mitigate the impact on the locality by increasing traffic – it’s meant to be spent on cycle provision, bus stops and pedestrian crossings etc). They keep telling us they have no money, so we wanted to find out. They have £4.5 million sitting there that’s accumulated over the past decade. They haven’t spent a penny despite requests from locals for crossings etc Yet they use their resources to have countless meetings, make lists and pay for consultants. We’ve told them what we want and need.
    The part that gets me is the reason is most likely, they only think about what increases traffic flow and have made cycling and walking dangerous and something to avoid, so now they say, why put a crossing in, no body walks there and why cycle provision there’s not enough cyclists. So the money just sits there, and they say there’s no money. They also will argue that provision takes road space away from cars.
    Their short sighted, uninformed incompetence is not unique and I’m sure if anyone did what we did with their local council, they would find the same thing – council’s claiming to have no money, but would find they are sitting on millions, they just don’t want to make them selves unpopular spending it the way it’s intended. As if drivers are the only people that vote.
    Your point is bang on but the whole country just can’t see it.

  3. rdrf says:

    I’m a bit disappointed in this post.

    As far as I’m concerned, reducing motor traffic is vital if we are to deal with climate change and a variety of other problems it causes.

    On top of this, just getting reasonable payment from motorists towards the costs they incur, plus enforcement of the law (which inevitably means banning some motorists) should reduce the amount of motoring anyway.

    Now, if some of the former motorists transfer to cycling, that’s just fine. The more the merrier. They may also walk, use public transport, share cars or stay at home – there are lots of ways in which people can reduce their car journeys.

    The point is that thinking about junction capacity in terms only of shifting some motorists on to bike is not enough.

  4. Part of the problem is current UK usage. When we were discussing Perne Road roundabout in Cambridge some people were insisting that at this roundabout the Dutch would not give priority to cyclists. I didn’t believe them.

    It took me a while to work out we were talking in different terms. I meant that cyclists would have priority on roundabouts at an inner-city residential location near schools. They meant that motor vehicles would have priority at a roundabout with that volume of motor traffic. Only problem is, you’re unlikely to find that volume of motor traffic in a residential / school area in Holland.

    • Or, I guess: the Dutch don’t have the complete solution for our junctions, because they don’t have our junctions. What would the Dutch do in Bedford? Who knows, they don’t have a Bedford. They might have had one 40 years ago. Which is not to say I don’t think infrastructure is the answer: I very much do. I’m not sure how to get there from here.

    • Jan says:

      Can you provide the volume numbers for that junction? I would be surprised if we can’t find something with similar numbers in the denser neighborhoods of the large Dutch cities. E.g. Amsterdam-Zuid has some small roads with very high volumes of car traffic, but still has reasonable cycling facilities (though not comparable with Groningen, but you’d have to start somewhere).

      • Survey data is here: http://www.cambridgeshire.gov.uk/NR/rdonlyres/451BDF8E-1DDF-4417-866B-E55D605DDE8D/0/trafficmodellingreportandappendices1and2.pdf appears to be 540 vehicles per 15 minutes (of which 125 cyclists) at peak.

        There are some other things to consider – you might be able to get more vehicles through the junction if more of them are cyclists, which should be the long-term goal. Also at least one regular route user suggested that you could reduce the capacity because this wasn’t the pinch-point of the network – it’s getting people to places faster where they will get stuck anyway.

        • Jan says:

          Dutch roundabout in Amsterdam-zuid. Not the best example of good cycle facilities (albeit better since the renovation last year, which isn’t visible in Google today):

          Roundabout: https://www.google.nl/maps?ll=52.349556,4.857572&spn=0.002176,0.003321&t=m&z=18

          Traffic between 16:00 and 18:00 on average working day (2008), according to http://www.verkeersprognoses.amsterdam.nl: ~3800 vehicles in 2 hours, which only includes motorized vehicles. From personal experience, I guess that the number of bicycles is about similar to the number of cars on a nice day, and a bit lower (but still over 50%) when it’s raining.

          But this is quite a large and complex roundabout (with a bus stop and tram lines). Most of the junctions with this capacity are normal junctions with traffic lights. However, it’s perfectly feasible to have the cyclists having priority, which is the norm for a roundabout within the city limits. If the cyclists can’t have priority, it shouldn’t be a roundabout in the dutch cities, since that would cause lots of confusion.

  5. Paul M says:

    I used to visit Southampton frequently, but in the last decade I think I have only been there twice – for my brother’s wedding at the registry office in the old town, and to take my daughter to look around Southampton University.

    On both occasions we drove there, and on both occasions (Saturdays) that was a huge mistake. I don’t think I have seen a more car-sodden city in quite a while, and that even includes London. For the Uni we would have been far better off taking the train to the nearby station at Swaythling or St Denys in the northern suburbs – even allowing for the out-and-back trip via Woking to change trains, it would have been quicker, and my blood pressure would not have got so elevated.

    On touring the Uni and some of its annexes and halls of residence, I was struck by how very few bicycles I saw around. I know it was out of term time but even so, I would have imagined that there would be more bikes locked up around the campus and indeed more bike parking than I actually saw. Mind you, the roads in that quarter of the city looked so hostile that I would not be surprised if no-one was willing to venture out on them. Perhaps that is why there is such a comprehensive Uni Bus network, with your hall-of-residence rent throwing in a complimentary season ticket.

    Mind you, I shouldn’t have been surprised. When I was doing some research into cycling in my old home town of Gosport, just a little further down the Solent and facing Portsmouth across the Harbour, I browsed the Census 2011 travel data which the Guardian put into their excellent interactive map. While Gosport itself had commuter cycle mode shares in the national top ten, and Portsmouth not that far behind, the Southampton hinterland – Swanwick and Whitely, where the Office for National Statistics and the National Air Traffic Service are based – has among the highest drive-to-work percentage and, correspondingly, low cycle mode share in the entire country. The A27 and M27 routinely jam solid in morning and evening rush hours, and traffic around the Southampton docks area is gridlocked.

    If anywhere needs to use more imagination, vision and courage about cycling, it is Southampton.

    • fonant says:

      The M27 was reasonably free-flowing even in rush-hour, some twenty years ago. Then they improved the M3 by cutting through Twyford Down (apparently they did this to reduce congestion!).

      The Wikipedia article on Twyford Down says “In 1994, a government committee concluded that building more roads encourages more traffic and that the way to ease congestion and pollution was to take measures to control car use rather than accommodate more. When Labour came to power in 1997, most of the road schemes were cancelled.”.

      Why, oh, why, do we keep repeating the same transport mistakes in this country?

  6. TomP says:

    As an update on the Bedford’s terrible design, it looks like it might be on hold thanks to pressure from motorcyclists: http://www.mag-uk.org/en/newsdetail/a7169 .

    This might be a good chance for cyclists to start putting pressure on the DfT again.

  7. Pingback: Using a flexible mode of transport to break rules designed for an inflexible mode of transport | As Easy As Riding A Bike

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