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3. What Effect Would it Have on Air Quality?

In order to work out the effect a scheme or action would have on air quality, information is needed on the following both with and without the scheme or action in place:

• Traffic flows;
• Traffic speeds;
• Fleet composition including age of vehicles;
• Geographical location of roads and properties;
• Background concentrations; and
• Meteorology

The DMRB screening method can be used to give an indication of the change expected for some actions such as changes to traffic flows, fleet composition in terms of vehicle type and daily average speed. The method is described in DMRB 11.3.1 and a spreadsheet is available to do the calculations. The data required for the spreadsheet are annual average traffic data and fleet composition, distance from property to each road centre within 200 m and annual average background concentration. However, the screening method is not suitable to assess the effect of some measures such as reducing congestion in peak hours, altering the speed by road lane, changing the proportion of vehicles meeting each Euro emission class or road lane usage.

More detailed calculations can be made by using a dispersion model. Detailed modelling can take account of, explicitly, variations in flows and speeds throughout the day, queuing traffic, local meteorological conditions, cuttings and embankments, road geometry, changes to age profile of vehicles, fleet composition and traffic description by lane. Further information is given in DEFRA's Review and Assessment: Technical Guidance LAQM.TG(03), which can be downloaded from www.defra.gov.uk/environment/airquality/laqm/guidance/index.htm.

3.1 Speed Changes

The graphs in Figures 1 to 5 below show the average vehicle emission rates of NOx, PM10, CO, non-methane hydrocarbons (NMHC) and CO2 in 2005 for a traffic flow with 5%, 10% and 15% heavy duty vehicles (HDV). The emission rates are tabulated in Table 2 for a vehicle fleet with 10% HDV.

Figure 1 - NOx Emissions by

Figure 2 - PM10 Emissions by Speed

Figure 3 - CO Emissions by Speed

Figure 4 - NMHC Emissions by Speed

Figure 5 - Carbon Dioxide Emissions by Speed

Source: DMRB Spreadsheet

The highest emission rates occur at the lowest speed of 5 km/hr for all of the pollutants. The lowest emission rates occur at 60-65 km/hr for NOx, 65-80 km/hr for PM10, 75-80 km/hr for CO, 90-100 km/hr for hydrocarbons and 65-70 km/hr for CO2. The increase with speed is greater for some pollutants, such as PM10, than for others, such as NOx where the speed curve is relatively flat.

The speed ranges over which emissions are within 10% of the minimum are shown in Table 3 together with the minimum emission speed range.

At low and high speeds, speed changes can have a more significant effect on emissions. Increasing speed from an hourly average of 5 km/hr to 10 km/hr could decrease emissions by 27% for NOx and 33% for PM10. Reducing the speed from 110 km/hr to 100 km/hr would decrease emissions by 6% for NOx and 14% for PM10. Reducing congestion is therefore important to improving air quality.

Some roads have considerable variation in speed from hour to hour with lower speeds during peak hours. The number of hours with congested traffic is likely to increase in the future as traffic flows increase. Table 4 contains traffic flow and speed data for a western section of the M25 together with the estimated hourly emissions in 2005 assuming that 10% of vehicles are HDVs. The daily emissions of NOx and PM10 are estimated to be 207.3 kg/km/day and 6.8 kg/km/day respectively.

If the speed had been assumed to be 112 km/hr throughout the day, the daily emissions of NOx and PM10 would have been estimated to be 225.9 kg/day of NOx and 8.0 kg/day of PM10, an overestimate of 9% and 18% respectively. However, if the daily average speed was used in modelling and the model results adjusted so that they were in agreement with the measurements, the overestimate would be removed. If future years were then modelled in the same way, the effect of any change to the speed variation throughout the day would not be reflected in the model results. As congestion is likely to increase in the future, this effect is likely to become more important.

It could be the case that one of the measures being considered in an action plan is to reduce the maximum speed limit to 100 km/hr. For the example discussed above, this would reduce emissions of NOx and PM10 by 1% and 2% respectively. However, if the speed had been assumed, incorrectly, to be a daily average of 112 km/hr originally, the emission reductions with a lower daily average speed of 100 km/hr would be estimated to be 6% for NOx and 14% for PM10, a larger reduction than would actually occur. Considerable care therefore needs to be taken when estimating the change in emissions that would result from a change in speed.

The estimated daily NOx and PM10 emissions for the scenarios discussed above are shown in Table 5.

Source: DMRB Spreadsheet

3.2 Vehicle Type

Emissions from a range of fleet average vehicles in 2005, travelling at 100 km/hr on a motorway are compared in Table 6. The table also shows how many times greater the emissions are for each vehicle type compared to a car, for example, an articulated HGV has 21 and 24 times more emissions of NOx and PM10 respectively than a car. Proposals that encourage a modal shift from HGV to rail could be effective in some situations.

Source: DMRB Spreadsheet
Note: The number in brackets denotes the emission equivalent in terms of the number of cars.

Emissions will decrease in the future as vehicles meeting more stringent emission legislation penetrate the vehicle fleet. The change in average emissions for a vehicle fleet with 10% HDVs, travelling on a motorway at 100 km/hr in 2005 is shown in Figure 6 for NOx and Figure 7 for PM10. The results are tabulated in Table 7.

Emissions are expected to decrease by about 8% per year due to the penetration of the vehicle fleet by new vehicles but this will be offset to a small extent, by the increase in traffic. Any measures which encourage the uptake of newer, cleaner vehicles will improve air quality.

Figure 6 - Change in Average Vehicle NOx Emissions with Time

Figure 7 - Change in Average Vehicle PM₁₀ Emissions with Time

3.3 Modal shift from cars to buses

The effectiveness in reducing emissions by persuading car drivers to travel by bus either through park and ride schemes or improved public transport will depend upon a number of factors. These include the emissions of each vehicle (determined by EU emission legislation), the number of cars that each bus effectively removes from the road, and any additional distance that the car travels to reach the bus. Table 8 shows the emissions at 50 km/hr for a 2005 fleet average car, 2005 fleet average bus and each Euro class bus.

The bus emissions are higher than those than from a car for all pollutants except carbon monoxide. Eighteen fleet average cars would need to be removed from the road network for each additional fleet average bus in 2005 if there were to be a reduction in emissions of all pollutants. However, the modal shift could have a greater reduction in emissions if newer, cleaner buses were used.

Source: Emission factors for fleet average car and bus - DMRB spreadsheet, for Euro bus - National Atmospheric Emissions Inventory, Emissions Factor Database http://www.naei.org.uk/emissions/index.php

Note: The number in brackets denotes the minimum number of fleet average cars in 2005 that would have to be removed from the road network for each additional bus, for there to be a decrease in emissions.