Tag Archives: Air Quality

20 mph and the Clean Air Zone

Bristol City Councils 20 mph review finishes today. In this consultation, the potential of 20 mph to help meet the National Air Quality Objectives (NAQOs) for NO2 and PM10 emissions, for which Bristol exceeds, has almost completely been ignored.

Instead, a Clean Air Zone (CAZ) has been proposed to tackle this as part of central governments Clean Air Framework. A CAZ would attempt to reduce air pollution by encouraging residents and businesses to purchase a cleaner car. A scrappage scheme has even been mooted as a way to kick-start this.

A CAZ might encourage some people to leave their car at home but it would have nowhere near the benefits of 20 mph limits.

One of the key infrastructure requirements for a CAZ is Auto Number Plate Recognition (ANPR) cameras. These are the same yellow cameras used along the M32 to enforce speed limits. They are one of the most effective measures for speed enforcement, ensuring drivers stick to the speed limits within the zone. Surely it would make sense to simply install these cameras to enforce the 20 mph speed limits that are already in place?

From Bristol Cycling’s perspective, 20 mph zones are only positive for Bristol. We recently highlighted the key benefits of 20 mph. In summary, 20 mph limits:

  • Reduce Collisions and lessen their severity when they do happen. A driver seeing a danger travelling at 30mph would have travelled 9 metres before they even pressed the brake pedal and another 14m to come to a stop. At 20 mph the overall stopping distance is half that. This makes a collision less likely in the first place. The kinetic energy of a 1419kg Nissan Qashqai car travelling at 20 mph is 15.7 Wh. If the vehicle didn’t stop in time, this would be the energy felt by the object it hits. Increase this speed by 50% to 30 mph and the kinetic energy goes up by 125% i.e. it more than doubles to 35.4 Wh. To put this in comparison, the energy of a 90kg cyclist at 20 mph is 0.9 Wh and a 70kg jogger at 5mph, 0.05 Wh. This video of an appalling collision in Wales demonstrates the huge amount of energy in a moving car.
  • A car braking from 30 mph to a stop puts more than double the energy through the brakes, tyres and roads than the same car stopping from 20 mph, increasing particulates (PM) from the brakes, tyres and road, contributing to poorer air quality. It also puts more strain on the road surface making damage and potholes more likely.
  • 20 mph has also been shown to reduce noise from vehicles. A car travelling at 20 mph instead of 30 mph can emit almost 10db less noise. 10 db is perceived by the human ear to be a halving of noise. Studies in Sweden, Germany and the UK, have consistently found 2—5 db noise reductions at the roadside from 30 kph (20mph) limits.
  • 20 mph Improves traffic flow and reduces journey time. Countless studies have found 20 mph reduces the concertina effect of traffic and reduces congestion.
  • Acceleration is the most energy-intensive stage of driving. Lower speed limits mean less time on the accelerator pedal and reduced fuel consumption. Considering a hypothetical a 16km journey with 25 stops and starts a car only accelerating to 20 mph will spend nearly a minute less on the accelerator than if it reached 30 mph. Although this does not directly correlate with emissions, a detailed study in London found (unenforced) 20 mph limits reduced NO2 air pollution concentrations at the roadside.

Over the past few weeks, Bristol Cycling has been campaigning hard to raise awareness of the 20 mph consultation. The criticism we hear time after time is that 20 mph isn’t enforced* and that “no one really travels at 20mph”. This isn’t a reason to scrap it, it is a reason to do it properly and see what the real benefits could be.

We’ve not heard anyone suggest the CAZ could be left to chance police patrols to enforce, so why has 20mph?

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Cycling vs the Electric Car

Electric vehicles have received a lot of press over the past few months. This furore has even led some to suggest that EVs are more efficient than food powered humans riding bicycles.

So we at Bristol Cycling have put together an unapologetically technical article in an attempt to shed some light on this.

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Fighting for more air

On this website we previously covered the excellent BBC documentary Fighting For Air. At the end of the programme, viewers were informed they could use an air quality postcode checker, enabling anyone in the UK to see how their area faired on a simple scale of 1 – 6. However, we delved into this for some areas of Bristol already covered by local council monitoring and were not convinced by the results.  

The checker considers only one of the 9 standards that apply to these 5 pollutants. This is the NO2 annual average, on a scale of 1 – 6. Where 6 is equal to an annual average NO2 concentration of 100µg/m3 or greater. This is slightly bizarre as any residential façade that exceeds 40µg/m3 is in breach of EU law.

Bristol Council has one of the most detailed and easy to use information maps of any council in the UK. One layer of this map is NO2 monitoring data from the diffusion tube network around the city. These are small tubes attached to lampposts, which record monthly average concentrations, which are then averaged over a year and verified by Defra. Comparing some of these results with the postcode checker gives some surprising results, shown in the table below:

The road featured in the documentary had an annual average NO2 of just over 40µg/m3. As mentioned, this is the legal threshold that applies at the windows of people’s houses. This level was clearly deemed necessary for action in the programme, yet some Bristol Streets with the same result are deemed to have “Good” air quality – a rating of only 1 out of 6.

Overall, the checker does not correlate with Bristol Councils monitoring results and it only refers to one of a host of pollutants that are part of the problem in Bristol.

Scientists knowledge of the impact of Air Pollution has on health is still in its infancy. Monitoring sites are few and far between and the technology for undertaking this is lacking. To pretend we have enough information to provide the public with a ‘one score’ ranking is misleading and hugely damaging to efforts of organisations such as Bristol Cycling, who are trying hard to raise awareness of the potential dangers of the current situation. It is also at odds with the precautionary principle.

We would point anyone in Bristol who wants an understanding of NO2 outside their house, to visit the council map mentioned above and select an Air Quality monitoring site on a road with similar traffic flows to theirs. Other pollutants can only be guessed at as monitoring is almost non existent.


A summary of the National Air Quality Objectives (NAQOs) that apply to Bristol:

Bristol, is one of the largest Air Quality Management Areas (AQMAs) in the UK, for Nitrogen Dioxide (NO2) and Particulate Matter <10µm (PM10). This means across the city pollutant concentrations of these have been breaching the National Air Quality Objectives (NAQOs), which are the government/EU target “safe” figures, across the city. These objectives  cover 14 different pollutants. Of these, 6 apply to Bristol. These are listed here.

  1. NOx – the precursor to NO2 that is emitted directly from combustion. Not in itself harmful to humans, but exacerbates eutrophication, which is the burden from excess nutrients on environments. The NAQO for ecologically sensitive areas (defined on this map) is an annual average concentration less than 30µg/m3.
  2. NO2 – NOx released by vehicles/gas boilers and other combustion sources reacts with sunlight/ozone and VOCs to form NO, NO2 and NO3. NO2 is harmful to human health. The NAQOs require annual average concentrations to be less than 40µg/m3 and hourly average concentrations should not exceed 200µg/m3 more than 18 times per year.
  3. PM10 – Particulates measuring less than 10µm. Bristol is an AQMA for PM10 meaning it has high levels. There is only one PM10 monitoring station in the city, so citywide levels are largely unknown. The NAQOs for PM10 is annual average concentrations less than 40µg/m3 and 24 hourly averages not to exceed 50µg/m3 more than 35 times a year.
  4. PM2.5 – Particulates less than 2.5µm. These particulates, when breathed into the lungs, are able to dissolve into the blood stream. Their impact on our health has not been researched extensively and is poorly understood. The  World Health Organisation (WHO) maintain that there is no safe level of PM10 and PM2.5 pollution. The UK NAQO is for annual average concentrations to be lower than 20µg/m3. A recent addition is a target to reduce concentrations 15% by 2020, an almost impossible achievement given that PM2.5 monitoring is almost non existent in the UK.
  5. Benzo(a)pyrene (BaP) – is released as a result of incomplete combustion, primarily wood smoke in urban areas. BaP falls under the Polycyclic Aromatic Hydrocarbon (PAH) family. It is a proven carcinogen. Annual Average concentrations should be less than 0.25ng/m3.
  6. Ozone – control of nitrogen oxides (NOX) emissions in the UK has led to an increase in ozone in urban areas. The NAQO is for an 8 hour average of 100µg/m3 not to be exceeded more than 10 times a year.

 

Fighting for Air

On Wednesday 10 January, BBC Two aired Fighting for Air, a documentary looking at how a local community in Birmingham, aided by a BBC team, set about tackling pollution on their local high street.

The street could have been a number of shopping streets in Bristol, with almost identical debates on issues involved.

The solutions put forwarded for reducing pollution were smoothing traffic flow by introducing green waves, replacing the parking bays with vegetation and handing out free bus tickets.

The natural reaction of some of the traders to removal of parking and reducing traffic was fear of reduced passing trade, but it was highlighted that improving the urban environment generally increases walking. Pedestrians generally spend more money in local shops than car drivers. An example given was the high street of Walthamstow in London. There, a Mini Holland scheme has seen vehicle numbers fall by 2,000 along the main shopping street and 10,000 in the local area, a prime example of traffic evaporation at work. The local butcher has seen an increase in trade, whilst the antique dealer thought his business had been negatively hit.

Back in the Birmingham high street, the most effective measure was proclaimed by the experts to be the Green Wave. This is the synchronisation of traffic signals in line with the speed limit of the road, to reduce the stop start flow induced by the signal setups in many of our cities. Stops and starts are the most fuel hungry stages of driving. Where drivers know a green wave is in place it also strongly incentivises smooth, steady driving below the speed limit.

The average speed of traffic in Bristol city centre in 2017 was 8mph. 10 – 12mph is recognised an easy speed at which anyone lightly turning bike pedals on a flat street will travel. Green waves at this speed could slash emissions in the city, improve the flow of traffic, result in calmer driving and most importantly harmonise the movements of cyclists and cars.

The measures were tested for a 12 hour period. Traffic volume remained the same, hinting that the free bus tickets weren’t so successful. However, over the period NO2 concentrations fell by by 10% and PM10 likely a result of the absence of parking, increase in vegetation and synchronisation of traffic signals. Birmingham City Council are now considering making these changes permanent.

Walking is best way to avoid pollution, but guess what’s worst…

Vehicle occupants are exposed to significantly greater health risks than those travelling by other modes. The excellent Essential Evidence series summarises key high quality evidence about transport and health and the latest one concludes that “vehicle occupants are, in fact, driving in a ‘tunnel’ of pollutants” with exposure levels significantly higher than walking, and also for cycling.

This is also the conclusion of a former government chief scientific adviser, Prof Sir David King, who says “You may be driving a cleaner vehicle but your children are sitting in a box collecting toxic gases from all the vehicles around you.”

Source: 162: Air pollution exposure among motor vehicle occupants – Travelwest

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