Just what is ‘smoothing traffic flow’?

In last week’s London Assembly motion on Cycle Safety (which you can view again here, around four hours in) Labour’s AM Val Shawcross made the following comments about Transport for London’s ‘smoothing traffic flow’ agenda, and how the policy is, in her opinion, weighted against the interests of vulnerable road users –

I do feel that, essentially, there is a design guidance within Transport for London, but that design guidance, the balance of judgement, at the moment is falling in favour of the Mayor’s smoothing traffic programme, and is disproportionately not representing the interests of cyclists and pedestrians. We saw that in practice, on the decisions being taken on the layout at Blackfriars Bridge. We can see it in the developing discussions about the layout of the Elephant and Castle; Vauxhalll; the King’s Cross layout. Many Assembly Members have been involved in discussions – as you have Chair – about road junctions in your own area, and you can see that the underlying judgements – the balance of the decisions that have to be taken – is wrong. TfL and the Mayor are not favouring cycle safety, and pedestrian facilities, as they now should be, in this modern age.

AM Andrew Boff, in a subsequent speech, refers back to this, saying –

Can I take some issue with Val Shawcross… on the ‘smoothing traffic’ agenda. Actually, ‘smoothing traffic’ is better for cyclists. It’s where traffic stops and starts, [that’s] where the dangerous moments are, for cyclists. Y’know, I don’t just get on a bike for photo opportunities. I do it every day. And actually, it’s where you stop and start that are the dangers. And actually smoothing that traffic flow will be safer for cyclists.

From Andrew Boff’s comment, you would think that the ‘smoothing traffic flow’ policy is simply an attempt to smooth off ‘spikes’ in traffic speeds.  To get technical, we might say that the standard deviation of the speeds of vehicles on London’s roads will decrease. That this is what Boff actually meant is further confirmed by some tweets he made later in the day, namely

Smoothing is not speeding


Are repeated periods of motorised acceleration and deceleration safer for cyclists?

He also made a further comment on the Cyclists In The City blog, namely

 As to smoothing, I’m afraid there was, in the debate, confusion between smoothing and speeding. The stops and starts of motorised traffic, apart from being more polluting, makes my cycle journeys less safe but that is, admittedly, just my experience. From the tweets I’ve seen, other people don’t agree with me but I’d like to see some evidence either way.

Which again suggests that Andrew Boff thinks ‘smoothing traffic flow’ is simply about eliminating ‘stops and starts’.

So we have a choice, it seems, between vehicles trundling along quite happily at a steady, moderate speed, or the hard acceleration, fast speeds, and subsequent hard deceleration of vehicles on the current road network. ‘Smoothing traffic flow’ is the cyclist’s friend, because it tries to create the former conditions.

Who wouldn’t want that option? Isn’t the Mayor’s ‘smoothing traffic flow’ policy something we can all sign up to?

Of course not. For the simple reason that this version of it I have just presented – Andrew Boff’s version – is not what ‘smoothing traffic flow’ is about, at all.

He is confused.

If you do some careful digging through the paperwork, motions and pamphlets released by the Mayor’s Office and Transport for London, it becomes quite clear that ‘smoothing traffic flow’ is nothing more than a strategy to reduce motor vehicle congestion. It has nothing to say about vehicle speeds – their acceleration, deceleration, speeding or otherwise. Where Andrew Boff got this notion from, I don’t know – at a guess, he has just taken the words ‘smooth’, ‘traffic’ and ‘flow’ and interpreted them to mean what he likes (which, interestingly, was precisely the strategy used during the public consultation on this policy).

Simply put, the Mayor is trying to reduce the severity of traffic jams, with the aim of  making journey times more predictable. Indeed, this is how the success of ‘smoothing traffic flow’ is being measured – by something called ‘Journey Time Reliability’, or JTR. If jams and queues are severe, then JTR decreases, and traffic is not ‘smooth.’

Andrew Boff’s version of ‘smoothing traffic flow’, therefore, is muddled, to put it mildly. The policy has nothing to do with vehicle speeds, at all – certainly not the instantaneous speeds he was referring to. Instead, ‘smoothing traffic flow’ is simply about mitigating the queuing at what TfL calls ‘pinch points’ in its road network. If you can get traffic (motor traffic, that is) flowing ‘smoothly’ through these places, then queues are minimised, and journey times are more reliable. The only, tangential, sense in which ‘smoothing traffic flow’ is about getting rid of ‘stop-start traffic’ is the way in which it attempts to reduce queuing times for motor vehicles.

‘Smoothing traffic flow’ cannot be about tackling ‘stop-start traffic’ in the sense Boff describes because the traffic is, of course, still stopping and starting. ‘Smoothing’ does nothing more than attempt to reduce the time for which it is stopped – but this is very different from getting rid of stopping and starting altogether.

The periods of acceleration and deceleration that Boff refers to – that he thinks ‘smoothing traffic flow’ will deal with – will, of course, remain, because quite obviously the Mayor is not eliminating stopping, at all. (Not even Boris would make so ridiculous a claim.) More precisely, he is attempting to eliminate gridlock, congestion and jams. But queues, quite naturally, will still form, and cars will still race from one queue to the next. It’s just that, if ‘smoothing traffic flow’ is successful, those queues will be shorter.

This is quite obvious when we look at what Boris himself actually says (pdf) –

I have launched a comprehensive effort to tackle the congestion conundrum and to smooth traffic flow so that Londoners no longer have to sit for hours fuming in the fumes.

‘Smoothing traffic flow’, therefore, is about tackling queues of motor vehicles. Indeed,  we see elsewhere that ‘smoothing traffic flow’ is, for Boris, a way of ‘preventing traffic jams.’

If there was any doubt remaining about the real purpose of ‘smoothing traffic flow’, the clearest evidence imaginable comes from the Blackfriars Bridge saga, and perhaps the early defining feature of the debate, that enflamed it – the 20mph speed limit.

If ‘smoothing traffic flow’ was really about eliminating ‘stop-start’ traffic in the conventional sense, and not, as it really is, about eliminating queues of motor vehicles, then at the very least Transport for London would have been keen to implement a 20mph speed limit across this bridge, to keep the traffic flowing at a more reasonable average speed.

In reality, of course, they took quite the opposite view, rejecting the proposed 20 mph speed limit, claiming that (pdf)

A permanent 20mph speed limit is not appropriate for Blackfriars. A temporary 20mph limit was introduced whilst the new station is being constructed. When completed, the new road layout, and in particular the new pedestrian crossings, mean that speeds during the morning peak are expected to be around 12mph through the junction – significantly slower than at present. For this reason a 20mph limit with additional signage and enforcement measures are not required.

Setting aside both a) the fact that pedestrian crossings have, in reality, been removed at Blackfriars, and b) the utter speciousness of this argument (quite plainly an average speed of 12 mph at the morning peak gives no indication of the maximum, and therefore most dangerous, speeds) the passage illustrates most vividly that Transport for London have no problem at all with traffic haring at crazy speeds from the queues at one end of the bridge to the other – the very definition of ‘stop-start traffic.’

Transport for London are not concerned in the least with addressing ‘stop-start’ traffic in the actual sense of keeping it moving at a more reasonable overall speed, as Andrew Boff would like to think.

All Transport for London are concerned about is minimising the length of the queues of motor vehicles at either end of the bridge, and by assocation the time spent queuing in those vehicles. The genuine ‘stop-start’ nature of the traffic simply does not enter into the equation.

It should now be entirely clear that every time Transport for London talk about ‘stop-start traffic’, what they are actually referring to are ‘queues of motor vehicles.’

To further illustrate, let me quote, and correct, Kulveer Ranger’s introduction from the same Transport for London pamphlet on ‘smoothing traffic flow –

The Mayor’s aim in managing the road network and smoothing traffic flow is to increase the reliability and predictability of  journeys, including tackling ‘stop-start’ traffic conditions queues of motor vehicles which increase emissions  of harmful pollutants.

Far more honest.

You can perform precisely the same correction every single time the phrase ‘stop-start traffic’ appears in the Mayor’s, or Transport for London’s, literature.

For if the intention really was to reduce emissions caused by ‘stopping and starting’ – hard acceleration and deceleration – we would have that 20 mph speed limit on Blackfriars, and indeed on nearly every road in London, without question; this would eliminate, at a stroke, a great deal of the ‘stopping and starting’, as vehicles travelling at the lower speed of 20mph would continue rolling towards traffic lights or junctions, rather than arriving at the queues there earlier, and sitting in them for  longer periods of time.

But Transport for London, as we have seen, are not keen on this lower speed limit. Indeed, quite the opposite – they seem rather set against 20mph limits on all the roads which they administer, and not just Blackfriars. The only conclusion that can be drawn is that they aren’t concerned with ‘stop-start traffic’ at all, but only with how lengths of queues of motor vehicles might impact about Journey Time Reliability. Which is, of course, what ‘smoothing traffic flow’ is all about addressing, not ‘stop-start traffic’. That’s why most of the measures being used to bring about ‘smoother traffic flow’ are designed to mitigate queuing times for motor vehicles at junctions. Including

  • Signal timing reviewsan issue I have written about before. Quite simply, the reallocation of traffic signal time, from pedestrians, to motor traffic.
  • SCOOT (Split Cycle Offset Optimisation Technique) – the careful management of traffic signals, to give more time to vehicles queuing from a certain direction.
  • SASS (System Activated Strategy Selection) – similar to SCOOT, this changes traffic signal timings according to a particular traffic problem.
  • Pedestrian countdown – again, more allocation of signal time phases to motor vehicles.
  • Signals removal – the stripping out of traffic signals that are deemed unncessary, or unjustified, for pedestrians, or ‘traffic’.

Needless to say, every single one of these measures is designed to reduce queuing time for motor vehicles, which is what ‘smoothing traffic flow’ is ultimately concerned with.

Although ‘achieving modal shift from car journeys towards more sustainable modes’ is listed as a ‘theme’ in implementing the smoothing traffic agenda, we have precious little in these documents about how that shift is actually going to be achieved. There’s absolutely nothing here, for instance.

Indeed, it is hard to escape the impression that ‘modal shift’ isn’t at all interesting for the ‘smoothing traffic’ bods at TfL, especially when, in their own documents, they describe how measures to achieve it have a ‘negative impact’ on ‘network capacity for motorised vehicles’ –

The causes of motor vehicle congestion are complex and actual traffic growth over the last few years has been small, partly due to investment in alternative modes (e.g. walking, cycling, and public transport improvements). Some of these modal shift measures, along with the impact of major utility infrastructure improvement programmes, and urban realm improvements, have impacted negatively on network capacity for motorised vehicles.

Measures to achieve modal shift, in other words, are antithetical to ‘smoothing traffic flow.’

An empirical demonstration of these attitudes within TfL came recently, with their open  willingness to prioritise the reduction of queuing times for motor vehicles over measures that could genuinely save the lives of vulnerable road users – even when those measures would quite reasonably be expected to achieve a modal shift.

So every time you hear the claim that ‘smoothing traffic flow’ is designed to ‘tackle stop-start traffic’, understand what is really meant by it.

Reducing queuing times for motor vehicles.

This entry was posted in 20 mph limits, Boris Johnson, Car dependence, Dangerous driving, Infrastructure, London, Road safety, Smoothing traffic flow, Speeding, Transport for London. Bookmark the permalink.

3 Responses to Just what is ‘smoothing traffic flow’?

  1. fred says:

    Of course, in the end, it doesn’t reduce queueing times for motor vehicles for long. As most road planners know by now, motor traffic tends to expand when capacity is increased, and fall away when capacity is removed.

    Click to access Disappearing_traffic.pdf

    Furthermore, by pushing junctions beyond their safe capacity for motorised traffic, ‘smoothing the traffic flow’ creates barriers to vulnerable users (who are also the most efficient users – people on foot and on bikes and in buses take up much less space than people in cars) – and so makes the junctions, in the end, much less efficient, overall.

    The most efficient (both in terms of capacity, and economically) design for a junction like Blackfriars would be one that ensured sufficient capacity for necessary local business and service private motor traffic, and then prioritised pedestrians, cyclists, and bus passengers.

  2. Tommi says:

    I find it most curious TfL would talk about wanting to smooth traffic and then design roads that encourage exact opposite. Efficient stacking depends on getting as many cars accelerate fast, get across a junction, and then slow down hard to wait the next set of lights. If instead you were to actually drive smoothly, i.e. at a speed that would allow you to get through next set of lights without stopping TfL measurements would show horrible inefficiency. Of course given the designed bottlenecks you can’t actually drive smoothly anyway given there’s more cars pushed through the junction that fit the lane afterwards, so the need to stop is pretty much guaranteed.

    Then again, I can’t recall seeing many people even attempting to drive economically in London.

  3. Pingback: A pilot project to make a Hampshire town more liveable | Pedaller

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