In our last blog we shared the results of the 2017 Council air quality monitoring, showing that Gosforth High Street had the highest recorded nitrogen dioxide (NO2) air pollution in the whole of Newcastle.
In this blog, we take a look at three key questions that came up in discussion following release of the 2017 air pollution figures. These questions are:
- Why has air quality on Gosforth High Street deteriorated?
- Does restricting traffic flow cause more pollution?
- What happens to traffic when roads are shut?
The better understanding we collectively have of the answers to these questions, the better chance there is to bring pollution down to legal limits as soon as possible.
1. Why has air quality on Gosforth High Street deteriorated?
The government estimates that 80% of roadside air pollution is generated by vehicle traffic so, even without looking at any data, it is reasonable to suggest that vehicle traffic has been the source of the additional pollution. The question then is whether the increase has been due to more traffic, or because traffic has been held up and is waiting longer and emitting more while it is waiting.
We do have some data that can help us to answer this. In the graph below the two lines represent average daily vehicle volumes on Gosforth High Street (red line / right axis) and air pollution (blue line / left axis) between January 2016 and December 2017. Each point represents the average of two months. The gap in the red line is because we don’t have vehicle volumes, measured by The Grove, for either January or February 2017.
It is pretty clear that the more vehicles there are on Gosforth High Street the worse the air pollution is, as the two lines follow each other quite closely, and that the increase in air pollution in 2017 compared to 2016 is almost certainly a result of more traffic using the High Street.
It is also clear that in both 2016 and 2017 air pollution was significantly worse than the legal limit of 40μg/m3 NO2 averaged over a year. We don’t have data for pollution at this location prior to 2016, as 2016 was the first year air quality was measured in this location, but based on vehicle counts we can be reasonably sure air quality wasn’t any better prior to 2016 as traffic levels have been declining on Gosforth High Street since 2010.
Diesel buses are often mentioned as a specific cause of pollution on the High Street and we hope that low-emission buses will form part of the Newcastle City Council Air Quality Plan due early 2019. This data suggests however, that having cleaner buses may not be sufficient by itself to resolve air pollution concerns on Gosforth High Street, as pollution was worse after August 2016 even though Arriva had introduced new cleaner buses about that time.
2. Does restricting traffic flow cause more pollution?
The Government’s 2017 Draft Air Quality Plan suggested “Improving road layouts and junctions to ‘optimize’ traffic flow” as a way of tackling air pollution. Improving traffic flow and reducing pollution were also described in the Council’s 2014 Air Quality Progress Report as objectives for the Salters Road / Church Road junction.
In the SPACE for Gosforth response to the Government’s Air Quality Consultation we asked that the the Government prioritise the most effective measures to reduce pollution, which according the Government’s own assessment did not include optimising traffic flow. The Government’s final plan acknowledged that there is “there is considerable uncertainty on the real world impacts of such actions“. This is because rather than reducing air pollution, changes that are designed to improve or optimise flow can lead to more traffic (and more pollution).
We can use the same traffic data to see what the effects were of the roadworks at Salters Road junction between about February and August 2016, and the effect of Killingworth Road being closed to traffic from July 2017.
This next graph shows week-day traffic in 2016 and 2017 compared to the average for the same month between 2013 and 2015. The dotted lines indicate months in which traffic was measured on fewer than half of the days in that month.
During the Salters Road junction roadworks (March-August 2016), when traffic was controlled by temporary traffic lights, there were 1032 fewer vehicles per day on average compared to 2013-2015.
During the second half of 2017, at the start of the Killingworth Road roadworks, the Council’s mitigation included “Timings on traffic signals on key routes programmed for longer green lights on routes that will see extra vehicles during peak periods” i.e. enabling more north-south traffic flow. During this period (August, October, November, December 2017) there were 1240 additional vehicles per day on average compared to 2013-2015. We have excluded September 2017 from this average as traffic volumes were counted on fewer than half of the days in September.
So when traffic was constrained (via roadworks) there were fewer vehicles and lower pollution, and when Killingworth Road was closed, and there were longer green lights for north-south traffic on the High Street, there were more vehicles and more pollution. It appears that rather than causing more pollution, the 2016 roadworks had the opposite effect and actually reduced pollution on Gosforth High Street.
To know whether the Killingworth Road effect is due to the mitigation that increased flow or due to rerouted traffic we need to answer our third question.
3. What happens to traffic when roads are shut?
In our blog about the Killingworth Road Metro Bridge Replacement from Jun 2017 we included this map below. In it we highlighted The Great North Road and Benton Lane as roads to avoid during the works as we thought these were most likely to be impacted by traffic re-routing as a result of the Killingworth Road closure.
Traffic volume data is available for these routes so we can now see what that impact was. This shows that the average weekday traffic volumes for The Great North Road and Benton Lane were almost exactly the same in the second half of 2017 as they were in 2016. i.e. there was no additional traffic on these routes when Killingworth Road was shut.
|Location||July – December 2016||July to December 2017||Change
|Great North Road (north of Hollywood Avenue)||28,298||28,218||-80|
|Salters Lane (Killingworth Road)||17,064||4,636||-12,428|
|Total (average weekday traffic)||67,436||54,855||-12,581|
Even if the extra thousand vehicles on Gosforth High Street are a result of the closure, despite traffic levels on the Great North Road were unchanged, that is still a significant reduction in overall traffic levels.
This is almost certainly an effect called Disappearing Traffic where, contrary to what most people might expect, reducing road capacity can lead to substantial reductions in overall traffic levels. Studies of Disappearing Traffic confirm that this doesn’t necessarily mean that people are travelling less, just that people have a range of choices for how and when to travel.
On Station Road there is a similar picture. Weekday traffic in the second half of 2017 is higher than previous years but by far less than you would expect if all 8,000+ vehicles previously using Hollywood Avenue had re-routed via Station Road, and not substantially more than it was 2013 or 2015.
The dramatic drop in traffic in September 2016 may be because of roadworks by Northern Gas Networks on Station Road. If so, it would be another example of constraining traffic flow leading to fewer vehicles and less pollution (25% less in September than the 2016 average).
Despite vehicle numbers in 2017 being higher than 2016, air quality measured on Station Road in the second half of 2017 was better (35.3μg/m3) compared to the same six months in 2016 (41.9μg/m3). This is most likely because the closure of Killingworth Road has meant far less traffic on Haddricks Mill roundabout.
Reducing Air Pollution
The data presented above suggests pollution is linked more to traffic volumes than flow, that constraining traffic leads to better air quality, and that closing Killingworth Road hasn’t made any great difference to traffic on The Great North Road or Benton Lane.
That’s not to say Killingworth Road has had no effect. For some people journeys will be worse, and others e.g. those that drive East-West across Haddricks Mill or use the 33 bus on Hollywood Avenue, which is no longer delayed by through traffic, will have better journeys.
It does mean we should be concerned by measures attempting to improve air quality by improving traffic flow, as they might just make it worse. The most effective way of reducing air pollution, according to the Government, is to introduce Charging Clean Air Zones (CAZ). These work by charging for, and therefore discouraging, trips made by the most polluting vehicles.
Newcastle City Council only has one month left to complete its plans to bring air pollution on Gosforth High Street and across Newcastle within legal limits in the shortest possible timescales. We don’t know for certain what this will involve but almost certainly it will need to include some sort of restriction on traffic if it is to have any measurable effect.
- Air Pollution Data is taken from Newcastle City Council’s annual monitoring reports using measurements taken on Gosforth High Street just south of Woodbine Road.
- Data on traffic volumes is from the TADU website. Gosforth High Street measurements are from near The Grove.
- We don’t have data for traffic queue lengths or journey times so don’t know how those have been affected, although it is a reasonable assumption that more vehicles using the Great North Road after the Salters Road junction works indicates that journey times were slower during the roadworks and faster afterwards, although it is also possible that the additional traffic negated that benefit.
- We do know that it now takes longer to cross Gosforth High Street on foot because pedestrian lights have been changed to prioritise traffic flow rather than crossing. The tongue-in-cheek Pedestrian Pain Index which estimates costs due to delays to walking journeys on a similar basis to the calculation for driving delays, is worth a read.
- These results are consistent with a world-wide survey of cities published in “Cities and Automobile Dependence” (1989) by Kenworthy and Newman. This concluded that the goal of “free-flowing” traffic (through such strategies as road widening) actually results in more fuel consumption and air pollution. For a good summary see the Walkable Streets blog.
- There are many similar studies that confirm that measures to improve flow will not reduce congestion either. This has been called the Fundamental Law of Road Congestion, which essentially states that any additional capacity for vehicles will quickly be filled negating any expected benefit.
- One of the most effective ways of reducing traffic delays while also improving air quality is via congestion pricing. In Stockholm, a where congestion pricing has received widespread public support, it has been estimated that without “congestion pricing”, children would have suffered 45 percent more asthma attacks. The video below by Jonas Eliasson gives a good overview of the approach taken in Stockholm and shows how public support for the approach grew significantly once it had been implemented.