Barbican Association Sustainability Group

Briefing paper on evidence to support the introduction of 20 MPH speed limit on City streets

At the Barbican Association AGM on 26 April 2012 there was overwhelming support for the introduction of 20 mph zone on City streets. This paper draws together some of the supporting evidence for reducing the speed limit.

The key argument for reducing the speed limit is that traffic becomes calmer. This means that acceleration and braking are reduced. Although the average traffic speeds in the City are about 10 mph, vehicles are speeding to get through traffic lights or braking at junctions, which has an adverse impact on emissions and noise and increases the risk of injury to vulnerable road users such as pedestrians and cyclists.

1. 20 MPH limit would improve air quality

The City has poor air quality; it is in breach of EU standards on nitrogen dioxide (NO 2)[1] and levels of fine particles (PM10) are high, although only Upper Thames Street is in breach of daily limits[2]. Emissions from road traffic comprised around 67% of all NOx emissions in 2011 in the City and 76% of PM10 particles. Road traffic emissions of PM10 particles arise from two main sources - from vehicle exhausts and from brake and tyre wear. In 2011 this was roughly equal, but brake and tyre wear is expected to contribute a greater proportion of PM10 particles by 2015 - see Table 1.

Table 1: Source of PM10 pollutants City of London 2011 and projected for 2015[3]

2011
% / 2015
%
Brake and tyre / 49 / 62
Exhaust / 51 / 38

Research has been carried out to determine how reducing the speed limit affects levels of emissions from both sources.

1.1 Reduction in exhaust emissions

Research in Germany[4] indicates that reducing speed limits from 50 kph (31 mph) to 30 kph (19 mph) reduces idling engine times by 15%, gear changing by 12%, brake use by 14%, and fuel use by 12%. Table 2 shows the relative change in emissions and fuel use for two different driving styles. Even aggressive driving under the slower speed limit produces lower emissions (but higher fuel use) than under the higher speed limit, although calm driving produces greater reductions for most emissions and net fuel savings.

Table 2: Percentage Change in Vehicle Emissions and Fuel Use with Speed Change from 50 kph (31 mph) to 30 kph (19 mph) (Newman and Kenworthy 1992)

emissiontype / 2ndgear
aggressive / 3rdgear
calm
% change / % change
Carbon monoxide / −17 / −13
Volatile organic compounds / −10 / −22
Oxides of nitrogen (NOx) / −32 / −48
Fuel use / 7 / −7

The results of a trial in Graz[5], Austria suggests that Carbon Monoxide (CO) and Hydrocarbon (H) levels may rise slightly with reduced speed limits but confirmed a 24% reduction in NOx. Where speed reduction schemes have smoothed the overall driving pattern, they have also been shown to produce slight reductions in CO and H levels.

This finding was confirmed by Jourmard et al (1992)[6] who used instrumented vehicles to investigate the relationship between emissions and speed and acceleration. The level of NOx emission was determined by the product of acceleration and speed. Traffic calming measures need not only to decrease speeds but also to smooth the overall journey for the driver. A 20 mph limit would achieve this by reducing acceleration and braking.

Metz et al[7] analysing traffic calming on the A9 Autobahn (although at higher speeds) found that calmer driving reduced particulate emissions by 36%.

Conclusion: a 20 mph limit would reduce all main pollutants emitted by vehicle exhausts and in particular NOx.

1. 2 reduction in particle emissions from brake and tyre wear

Measuring emissions of PM10 particles from tyres and brakes is very complex and there has been little research on it in the context of urban, low speed roads. This is recognised by the London Atmospheric Emissions Inventory (LAEI). They point out that:”In working towards achieving future air quality objectives, tackling tyre and brake wear emissions will be important, given the large proportionate contribution to PM10 concentrations that they now make. Reducing the number of vehicles on the road, encouraging the use of lighter vehicles and encouraging smoother driving techniques may help reduce tyre and brake wear emissions.[8]

Conclusion: it is reasonable to suppose that reduction in acceleration and braking resulting from reduced speed limits in an urban context would reduce particulate levels including PM10 particles.

1.3 Encouraging zero emission walking and cycling

There are also intangible benefits resulting from reduced speed limits. Studies have shown that in areas with 20 mph speed limits the number of people walking or cycling for short journeys increases. By encouraging more walking and bicycling and reducing use of taxis and buses for short journeys, traffic calming usually reduces the number of trips, trip starts, and Vehicle Miles Travelled (VMT). Walking and cycling levels rose by up to 12% after Bristol introduced its 20 mph limit.

2. Reducing the speed limit would reduce noise

Traffic is measurably quieter at slower speeds. In urban areas with speeds of between 20 and 35 mph, reducing speeds by 6 mph would cut noise levels by up to 40%. At lower speeds, engine noise becomes more dominant than ‘rolling’ noise (tyres, body), with acceleration accounting for 10% of all traffic noise – so, again, calming will quieten traffic.[9]

In 2008 in Munich speed limit reductions were evaluated for one of the busiest roads in a densely populated area. The speed limit reduction from 60 kph (37 mph) to 30 kph (19 mph) was predicted to produce an average 3 decibel (dB) reduction with no change in traffic flow or composition.[10]

Conclusion: a 20 mph limit should reduce traffic noise, particularly at night when congestion is less and traffic speeds increase.

3. Speed reduction reduces road casualties

In the City in 2011 there were 419 road casualties and casualties of both pedestrians and cyclists are on a significantly rising trend. Reducing the speed limit to 20 mph is likely to have a significant effect on the level and severity of road casualties.

Many UK cities have introduced 20 mph zones in residential streets (Bristol, York, Liverpool). Cambridge, Oxford, Newcastle upon Tyne and Coventry have introduced City-centre 20 mph limits. Considerable research confirms that such zones significantly reduce the level of road casualties.[11]

In 2009 the BMJ published a study of injuries in traffic speed zones in London[12]. Grundy found that “the introduction of 20 mph zones was associated with a 41% reduction in road casualties, after adjustment for underlying time trends. The percentage reduction was greatest in younger children and greater for the category of killed or seriously injured casualties than for minor injuries. There was no evidence of casualty migration to areas adjacent to 20 mph zones, where casualties also fell slightly by an average of 8.0%”.

The relationship between speed and injury severity is particularly critical for vulnerable road users such as pedestrians and cyclists. For example, pedestrians have been shown to have a 90% chance of survival when struck by a car travelling at 30 km/h (19 mph) or below, but less than 50% chance of surviving an impact at 45 km/h (28 mph)[13].

4. 20 MPH and average speeds and congestion

It has been suggested that reducing the speed limit would increase congestion and journey times. There is little evidence for this. In fact, lower speed limits reduce that level of congestion. Drivers cut their spacing as braking distances contract: shorter gaps mean more vehicles can use the available road space, reducing standing traffic. Furthermore, motor traffic volumes decrease, since slower speeds encourage active, sustainable and shared travel.

Filtering at junctions also becomes easier so junctions work more efficiently and queues reduce.

Finally, buses operate more efficiently, and bus journeys become more reliable. The reduced length of queues both cuts bus journey times directly, and allows the same fleet of buses to provide a more frequent service.

1

BASG June 2012

[1] City of London Air Quality Strategy 2011-2015. Nitrogen dioxide limit is 40μg/m3. The roadside sites Walbrook Wharf, Beech Street exceed the limit by a significant margin. The hourly limit of 200μg/m3 should not be breached by more than 18 hours in any year.

Background sites comply with this limit, but air quality at the roadside exceeds it by a significant margin. Walbrook Wharf recorded a total of 955 hours above 200μg/m3

in 2009.

[2] Ibid, p.26

[3] CERC, “Air Quality Modelling for the City of London Corporations: Final Report, 3rd February 2011.

[4] Quoted by (Newman and Kenworthy 1992, 39–40), based on research in Buxtehude.

[5]Sammer and Wernsperger (1995) describe an extended trial of 30 km/h zones; 170 test drives were conducted in the network using an instrumented vehicle which measured speed and distance travelled every second. This information was combined with emission estimates from previous studies to produce an estimate of the environmental effect of the speed limit reductions. They found that in the 30 kph streets there was an increase in Carbon Monoxide (CO) emissions of 3.8% and an increase in Hydrocarbons (HC) emissions of 0.5% but there was a 24% reduction in emissions of NOx.

[6] Jourmard R., Hickman J.A., Nemerlin J And Hassel D. (1992) Modelling of

emission and fuel consumption in urban areas- Final report. INRETS report No. LEN9213,

INRETS, Bron, France.

[7] Metz, N., Schlichter, H. and Schellenberg, H. (1997) Positive effects of a traffic control system on fuel consumption and exhaust emissions on the German A9 Autobahn. Int. J. of Vehicle Design. Vol. 18, Nos. 3/4 (Special Issue), pp. 354-367. Quoted by Working Paper R 1.2.1 “The effect of speed on noise, vibration and emissions from vehicles”, VTT Communities & Infrastructure (VTT, Finland), p. 20.

[8] The London Atmospheric Emissions Inventory (LAEI), compiled and distributed by the

GLA

[9] Mitchell, 2009, Speed and Road Traffic Noise, UK Noise Association

[10] Ibid

[11] A web search did not reveal any information that cast doubt on the benefits of introducing 20 MPH zones with one exception. A study by Atkins Transport Planning & Management for the Department for Transport in 2010 analysed the Portsmouth 20 MPH scheme. Atkins found in the 2 years after implementation overall casualties reduced by 22% although the level of Killed and Seriously Injured (KSI) rose from 19 individuals to 20 per year. Atkins emphasised that because of the small numbers of KSI no inference could be derived from this increase. Atkins’ overall conclusion is similar that for other 20 MPH schemes, namely that “early figures suggest that the implementation of the 20 mph Speed Limit scheme has been associated with reductions in road casualty numbers. The scheme has reduced average speeds and been well-supported during its first two years of operation.” Nevertheless the report was selectively quoted by sections of the motoring press who drew negative inferences from the report.

[12] C. Grundy, “Effect of 20 mph traffic speed zones on road injuries in London, 1986-2006: controlled interrupted time series analysis”, BMJ 2009

[13] World Health Organisation Fact Sheet