Making sense of Traffic Flows
- Average daily traffic - Local Statistics (2018)
- Average versus "Peak Flow" - traffic is "lumpy" rather than smooth.
- "Service Rate" - the most traffic that can enter and leave a section of road without congestion building up - an introduction to the idea of Queueing Theory
- Links between traffic flow and pollution.
- Links between wind direction and concentration of pollutants (Canyon Effect)
Count point at 118 London Road, Greenstreet.
Counting traffic along the A2 between Vincent Rd and Faversham Rd [Newnham/Doddington]
|AADF Year||Link Length km||Link Length miles||Pedal Cycles||Motor cycles||Cars Taxis||Buses Coaches|
|Light Goods Vehicles||V2 Axle Rigid HGV||V3 Axle Rigid HGV||V4 or 5 Axle Rigid HGV||V3 or 4 Axle Artic HGV||V5 Axle Artic HGV||V6 or More Axle Artic HGV||All HGVs||All Motor Vehicles (average per day)|
Traffic is "lumpy".
- Traffic is heavier during commuting times;
- Sundays see fewer vehicles than 'work days';
- Seasonal holiday traffic in the summer is not the same as winter holiday traffic.
Under ordinary conditions we see "peak traffic" in mornings and evenings as people go about their day to day lives (e.g. working near and far, school runs). Through the rest of the day we shop, visit, take deliveries, and so on. So lunch-times can show a lesser peak in traffic.
'Lumpiness' in traffic flows can also be made worse by, for example:-
- parked cars, deliveries, bus stops, wider HGV and other agricultural loads that narrow lanes for on-coming traffic;
- junctions (people slow down or stop until it is safe to move); and
- traffic lights/pedestrian crossings which block traffic whether or not there is much traffic.
- Abnormal additions:-
- motorway accidents and maintenance divert to the A2;
- A2 accidents and maintenance;
- new large-scale developments (construction traffic) leading to long-term increases in cars and deliveries (LGV/HGV);
- seasons in farming - e.g. harvesters, tractors and trailers and delivering crops to cold stores and processing plants; and
- brickearth excavation builds up HGV traffic levels.
Statistics vs reality
Statistics along the A2 in Greenstreet:
- At one extreme, if the whole "average daily" traffic flow of 13,140 (2017) arrived at the same time, the tailback would be enormous.
- At the other extreme, if the 'average' number of road-users were spread evenly every hour throughout the whole 24 hours of a day (and night) you would see 13,140 divided by hours in the day (24) to give 547.5 road-users an hour (or 9 road-users a minute). This would look and feel quite different!
Of course, both these extremes are nonsensical and 'reality' falls somewhere in between. "Real-world" road-use is variable. However, "averages" do give us a sense of the scale of traffic using a stretch of road.
Every road has a maximum number of road-users that it can handle smoothly at any given time. That is to say, the number of vehicles entering a section of road flows freely to leave that section of road without having to change speed. This is often referred to as its "service rate".
- If the number of vehicles using a road is a lot less than the "service rate" - problems begin as soon as the number of vehicles joining a stretch of road gets closer to the "service rate" for that road.
The 'tipping point' may be different for each direction of flow. If all the obstructions are on one side of the road, then the other side has right of way.....and congestion builds in the other directiion and, eventually, the congestion extends in both directions.
- The change between 'freely flowing' and 'congested' happens very quickly with only a small increase in vehicles creating a massive change to the flow rate.
Congestion happens quickly! With each new development and expanded transport depot, congestion becomes more likely and will worsen for longer periods as even more traffic arrives.
For the A2/Greenstreet, most static obstructions (parking and deliveries) are on the east-travelling lane (to Faversham). Traffic in both directions are obstructed by traffic turning into and out of Station Road, Lynsted Lane and Frognal Lane. Junctions slow traffic flow down, sometimes to a halt. A review by Highways England (April 2019) confirms that traffic lights on busy roads can make matters worse across the whole day.
- Fuel. Slower traffic burns more fuel for longer and less efficiently. To cover the same distance, slow congested cars use lower gears that make an engine pump out more pollution than they would in fourth gear in free-flowing traffic.
- Fuel. Stationary traffic pumps out pollution from combustion until it leaves the 'stuck' piece of road.
- Friction. Moving traffic rubs rubber compound tyres against the tarmac road surface and both surfaces wear down and create very fine particles of damaging particulate matter (P.M.).
- Friction. Repeated braking as vehicles stop and start through traffic queues release even more very fine particles from brake-pads and discs, including metals.
PM2.5: The most worrying types of pollution created by road-users falls into a the very fine-sized particles - the Particulate Matter of 2.5microns size or smaller (PM2.5). Particles of this size pass deeply into our bodies.
In our AQMAs, SBC is measuring only ONE type of PM2.5 - Nitrogen Dioxide (NOx). NOx is created by combustion of fuels and is falling nationally as engines become cleaner and electric, hybrid and hydrogen fuels take over.
SBC and other Borough Authorities miss out all the other PM2.5 particles that increase as vehicle numbers grow and as congestion increases. You can visit a page that talks about PM2.5 here.
The Canyon Effect. Pollution from ALL traffic is delivered to our lungs in the spaces used by pedestrians, buggies, wheel-chairs of all types and other road-users stuck in a queue using ventilation (windows or fans). Some of that pollution also finds its way into homes, shops and offices - that pollution is with us all day. Other sources of background PM2.5 include agriculture.
Under certain conditions, pollution is trapped between buildings on both sides of a road in a "canyon" and is concentrated even more at pavement level because each passing/stationary vehicle adds its pollution to what is already there from previous vehicles. This "Canyon Effect" is what we have in Ospringe, Teynham/Lynsted, Sittingbourne and Newington (the AQMAs).
The "Canyon Effect" only works when the prevailing winds blow across the line of buildings/road. The A2 runs east to west; prevailing winds mostly blow from the south. The wind, passing over our roofs trap and churn the air between the buildings. This effect works all day and night to concentrate pullutants - much, much more dangerous at times of heavy traffic and congestion.
Planners and Developers talk about reducing our dependancy on cars. At times, each of these ideas has been aired and can be seductive. But the scale of the problems with the A2 and our lanes may not dent the number of cars.
- Cycling? More cycles will be used along the A2 if only cycle stores are included in new homes? The A2 is a dangerous place for cyclists today. More cycles tomorrow doesn't make sense. Many cyclists use pavements to stay safe (but risk injuring pedestrians).
- The statistics (above) tell us how unequal the behaviour of road-users is.
In 2017, each day the average number of cars was 10,542; in the same period there were only 30 cyclists (down from 118 cyclists in 2000). There is no space to provide cycle lanes along the A2, which barely copes with two vehicles to pass eachother in the built-up areas.
- The statistics (above) tell us how unequal the behaviour of road-users is.
- Car sharing? A seductive idea, good in principle, but not everyone runs their lives to the same clock.
- School 'walking buses'? Another excellent idea. But many people prefer to use their car for convenience and safety and/or because they then go on to drop children at other schools, shop or work. Car sharing for delivering children to schools has potential for reducing congestion around schools.
Cars / vans per household
|2002 /03||2003 /04||2004 /05||2005 /06||2006 /07||2007 /08||2008 /09||2009 /10||2010 /11||2011 /12||2012 /13||2013 /14||2014 /15||2015 /16||2016 /17|
|Region of residence:-|
|Rural-Urban Classification2 of residence:-|
|Urban City and Town||1.13||1.15||1.16||1.19||1.20||1.20||1.18||1.18||1.20||1.17||1.17||1.18||1.18||1.21||1.25|
|Rural Town and Fringe||1.32||1.30||1.32||1.36||1.37||1.40||1.43||1.43||1.41||1.39||1.37||1.37||1.39||1.41||1.43|
|Rural Village, Hamlet and Isolated Dwelling||1.63||1.60||1.62||1.63||1.62||1.61||1.68||1.75||1.73||1.67||1.67||1.74||1.77||1.79||1.76|
|1. Two survey years combined, e.g. 2016 and 2017. A survey year runs from mid-January to mid-January.|
|2. For more information on Rural-Urban Classifications see:|
On the face of these figures, people living in rural England have 45% more cars/vans per person than the national average. This reflects the reality of poor bus services and a lack of convenient access to services (shops, GPs, libraries, post offices,etc).