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Cutting Carbon emissions is not just about cleaner vehicles

A personal perspective from Former Transport Minister and Strategic Advisor to Clearview Traffic Group, Dr Stephen Ladyman.
As I said in my last blog, the roll out of Ultra Low Emission vehicles will play a big part in reducing greenhouse gas emissions from road transport but, on the basis that every little helps in the battle against climate change, we need to look at other solutions.

The way we use vehicles also plays a big part in determining how much carbon dioxide they produce – and that is where traffic management strategies come in.

In their work in Southern California Barth and Boriboonsomsin [1] showed us why. In ideal conditions the emission of CO2 from cars is predicted by the formula: ln(y) = b0 + b1X + b2X2+b3X3 + b4X4 (where Y = CO2 emissions (gr/mi) and x is average trip speed in mph), shown in the graph as the red line.

From that formula we can see that cars travelling below or above the optimal speed range for minimising CO2 output (approximately between 30mph and 55mph) produce significantly more CO2 than when they are used at optimal speed.

Unfortunately cars don’t operate in an ideal world, they seldom travel at the optimum efficient speed for the design of engine and they have to start and stop, slow down and speed up.

The actual emissions in the real world are shown in the graph as the blue line.


That means that any strategy that encourages vehicles to operate in a way that moves the blue curve in the graph closer to the red, or travel more often in the optimal speed range, contributes to cutting carbon emissions.

Congestion mitigation strategies (ramp metering, incident detection and management, adaptive signal control, journey time monitoring and smarter parking to name just a few) and traffic smoothing techniques (such as variable speed limits and managed motorways) will all keep traffic flowing and make it more likely that average speeds are closer to or in the optimal range.

Barth and Boriboonsomsin also predict that when congestion reduces average traffic speeds to a range below the optimum range, that is between 10mph and 30mph, a 45% CO2 reduction can be achieved by smoother traffic flow.

If speeding up congested traffic can have a positive impact on carbon emissions, slowing traffic down when it is running above the optimum range also has a major impact.

The same paper also shows that reducing traffic speeds by 10mph when the average speed is well above the optimum range can cut carbon emissions by as much as 70%.

In other words, strategies that cut congestion and smooth traffic flows cut CO2 emissions and will help the Government meet its Climate Change Act commitments; and they also have a positive effect on the economy by reducing business costs which is a big bonus.

As a by-product they are also popular with the public – no-one ever lost an election by cutting traffic jams and the time of a voter’s commute.

On the other hand speed enforcement strategies can also have an important impact on carbon reduction targets but are far from popular with the public.

And at the end of the day, traffic management strategies have to be a compromise between carbon reduction, congestion mitigation, cost and politics – just how much of a compromise will be the subject of my final blog in this series.

To find out more about how Clearview Traffic with congestion mitigation, please get in touch here.
Uploaded 30/04/2015