Cutting congestion ‘is’ easy, as is cutting pollution

This article was published in 2015, in Newsletter 123.

The benefits of travelling on Park & Ride buses are significantly undermined by current congestion.
Image as described adjacent

I’m just old enough to remember London smogs, and the more general severe fogs of the early 50s. The various Clean Air Acts, firstly in London in 1954, and more generally in 1956, vastly and easily reduced these smogs and fogs, which had caused huge disruption to daily life and thousands of deaths.

We don’t even need Parliamentary Acts to reduce our local congestion and pollution, just a better understanding of the relationship between FLOW and DELAY. Understand that, and then the ways to reduce congestion, and hence much of the pollution, become clear and easy.

Research in the 1960s, almost as old as Clean Air Acts, into motor vehicle congestion showed that in a congested network, reducing traffic flow by 5% reduced congestion by 50%, and that reducing it by 10% reduced congestion by 90%.

Many find this an unbelievable statistic, but easily obtainable local evidence shows this is true in practice.

For a number of years I’ve asked small audiences, ‘How much does traffic drop in the peak hours over a half-term week?’ I’ll usually get some who say by more than 50%, and many who say by more than 30%. Few would dare to suggest it was as little as 10%. Yet the cast-iron evidence is available via the Cambridge County Council’s own Automatic Traffic Counters on several main radial roads in Cambridge.

Time period 07:00-08:59 Milton Road vehicles Huntingdon Road vehicles
Mon-Fri Out In Out In
Mean of seven term-time weeks 1064.0 1681.4 750.9 1671.6
Half-term week 996.2 1544.8 725.4 1654.0
Change in flow 6.4 8.1 3.4 1.1

Queuing theory is complex but the results are simple. As the flow nears capacity, delays increase exponentially. As users we clearly see, and are frustrated by, the delay and think that when travelling on a road free of congestion that the flow must be much lower. But the relationship between ‘flow’ and ‘delay’ is far from linear, with a clear ‘elbow’ effect. Delays rise little until around 80% of the road capacity is used, but then rise rapidly as flow increases. Clearly no more than the maximum traffic can get through a junction or link, so delays really do then escalate!

Commuter parking on Adams Road.
Image as described adjacent

Of course, this being Cambridge, some blame ‘cyclists’ for congestion, and cite worse congestion on wet days, claiming that it is those who normally cycle, but drive on wet days, who are the cause. Again, evidence from elsewhere, and common sense, disproves this. Congestion in places such as Sheffield, where cycling is rare, is worse on wet days. It is, in fact, the careful driver who is to blame for reducing the capacity of the road and hence increasing congestion. That caution in leaving a slightly larger gap lest braking be less effective is just sufficient to reduce the capacity of a road or junction.

Snow or ice is the extreme example. Queues can last for hours, not because there is suddenly more traffic but because, with good reason, almost everyone becomes ultra-cautious, and those who are not create even more delays!

If you thought traffic volumes needed to be reduced by 30% or more to eliminate congestion, that would be an almost unachievable objective. But a 10% reduction should be easily achievable. I’ve explained the cause, but what is the solution?

How to reduce congestion

If just one in twenty drivers were to use the bus and one in twenty were to cycle, congestion on normal days would be manageable. Of course there is an ever-increasing demand for travel, with more jobs and more housing in and around Cambridge, so other measures are needed, many of which are discussed in the Transport Strategy for Cambridge and South Cambridgeshire published last year.

There do need to be carrots and sticks, but we could have a virtuous circle whereby reduced private motor traffic makes buses more reliable and cycling more pleasant. How do we achieve that, and without the years of huge disruption that would caused by say, reconstructing Milton Road, or charging every motor vehicle that moves within Cambridge with some difficult-to-collect cash?

If just one in twenty drivers were to use the bus and one in twenty were to cycle, congestion on normal days would be manageable

Restrict commuter parking

Many European cities have reduced the availability of commuter car parking, especially that which is free to the users. The easy availability of parking can be directly related to the length of traffic queues on a typical day.

A Cambridge example would be, say, Adams Road. Overnight there might be fewer than ten cars parked, but during the day the figure will be around 120 (I’ve counted them). If parking were banned on most of this road, 100 fewer cars could enter Cambridge. One hundred cars make a queue of one kilometre. So the queues into Cambridge would be shorter by that amount. Furthermore, if people know they cannot park in certain streets they won’t drive around looking for spaces.

The above is a model, not a direct suggestion. Because of the ‘churn rate’ of car trips and jobs, many of which are short-term, few individuals would be directly disadvantaged if, say, 5% of free commuter spaces were removed each year, yet the direct advantages would be great. Congestion would be reduced, buses would be more reliable, even without bus lanes, and selective removal of such free parking on minor residential roads would make both cycling and walking more pleasant, hence encouraging these sustainable modes.

Gating or queue relocation

The idea of ‘gating’ or ‘queue relocation’ has also been raised. This, like reducing commuter car parking, will only be seen to be a workable option if the ‘system’ of flows, delays, and queues is well understood. As anyone who uses the Catholic Church junction in the peak will know, this junction becomes very inefficient because of ‘blocking back’, where queues from adjacent junctions obstruct vehicles (even cycles) that would otherwise have a clear path. Urban Traffic Control Systems such as SCOOT have long had the ability to reduce such issues, by rationing traffic into a link so as to reduce such blocking. Gating is just an extension for a section of road with several junctions and links between them. Where it is simple and cost-effective at the start of an otherwise congested radial to introduce a bus lane, it is provided. Other traffic is ‘regulated’ into the otherwise congested sections, and buses can then advance through links and junctions without the risk of being seriously delayed by queuing private cars on those sections they share.

I hope I have shown above that the difference in the numbers of cars on our radial roads between ‘congested’ term-time weeks and uncongested half terms means you don’t need a huge stack of hundreds of cars to achieve this, as the difference of flows on Milton Road over all of a two-hour period is only 140.

In addition, because the congested junctions do not get obstructed, traffic may well flow more smoothly, and the ‘gating’ allows an increased amount of normal traffic to enter, so that without any modal shift journey times should fall.

This needs no bus lanes on narrow urban roads, and other traffic is only ‘gated’ at all on those (fewer that 150?) days when serious congestion would otherwise occur.

I haven’t talked about pollution. This has been much in the news, with general agreement that levels are far higher than has been recognised, and much is caused by private cars stuck in congestion. Modern Euro 6 hybrid buses will produce less nitrous oxide than even a small diesel car. Once the implications of pollution caused by motor vehicles are better understood, access restrictions may have more public support.

We can never remove the threat of congestion, and to reduce those days when a serious crash closes a ‘Strategic Road’ around Cambridge, and causes chaos on local roads, we just need a better standard of driving.

None of these issues of major concern can be considered in isolation. Commuter car parking, bus delays, pollution, a better standard of behaviour by all road users and making cycling a viable option for ordinary people are all interlinked. To solve them at minimum cost we need to tackle them in a holistic and systematic way. Not as isolated problems.

Jim Chisholm