12.1) How do you estimate my car's CO2 emissions?
12.2) What assumptions do you make about my cars' CO2 emissions?
12.3) What assumptions do you make in estimating the CO2 emissions produced for your public transport and air journeys?
12.4) What assumptions do you make in comparing CO2 emissions for car journeys with public transport journeys?
12.5) What assumptions do you make about bus and coach journeys?
12.6) Why do you use energy efficiency ratings?
12. CO2 Information
12.1) How do you estimate my car's CO2 emissions?
To estimate the CO2 produced by a car journey we first calculate the distance travelled along the planned road route. We calculate the total distance by adding up the distance for each small segment of the journey along the road network. Usually each segment is made up of the distance between adjacent road junctions. We then estimate the fuel your car will use travelling along that route.
To do this we take account of how many miles per gallon your car does and the predicted congestion. We also account for urban driving where there is a higher frequency of road junctions. This enables us to factor in the stop start nature of driving in a built up area which uses more fuel and produces more CO2 than doing a journey of the same distance on a rural road or motorways.
Once we have calculated the fuel used we convert this to the CO2 produced using a conversion factor depending on whether the fuel you use is petrol or diesel.
12.2) What assumptions do you make about my cars' CO2 emissions?
To find out the amount of CO2 your car produces we calculate the amount of fuel used for a specific journey. We convert this to the CO2 produced using a conversion factor depending on whether the fuel you use is petrol or diesel
You can enter your car's fuel efficiency if you know it. This will mean that the calculations we provide are specific to the fuel efficiency of your car.
Many people are not sure of their car’s fuel efficiency. If you do not know this we can provide an approximation of the fuel efficiency and CO2 emissions for your car if you tell us whether it has a small medium or large engine and whether it uses petrol or diesel. If you don't tell us we will assume that you have a medium sized petrol engine car.
Click here to see the assumptions we make about the miles per gallon for different sized engines 
.
12.3) What assumptions do you make in estimating the CO2 emissions produced for your public transport and air journeys?
First we calculate your journey distance
Second we multiply this by a specific factor for the particular modes you will use. This gives the CO2 emissions for your journey.
Currently these figures are general averages for each type of public transport. Each figure assumes an average number of passengers for the typical sort of vehicles used when travelling by the particular type of transport. The figures come from Department for Transport (DfT) and have been agreed with Department for Environment Food and Rural Affairs (DEFRA). Click here to see these figures .
We do not always know your exact journey distance by public transport so we have to make a number of assumptions.
For a bus or coach journey, or the bus section of a longer journey, we know the distance, as the crow flies, between the stop you will get on the bus and the stop you will get off it. We therefore take the distance as the crow flies and multiply it by 1.25 to give an estimation of the bus journey’s length. We reached this figure by taking a sample of urban, semi urban and rural bus journeys and comparing the distance as the crow flies with the road distance calculated by the car route planner.
For a rail journey, or the rail section of a longer journey, we know the distance between each intermediate stop on the journey. We add these distances between stations up to give us the total distance for the train part of your journey.
For a plane journey we know the distance as the crow flies between the two airports you will fly between. We take this distance and multiply this by 1.09 to take account of the additional circling that a typical plane will do around take off and landing. This figure comes from Department for Transport (DfT) and has been agreed with the Department for Environment Food and Rural Affairs (DEFRA).
Check Journey CO2 calculations are based on point to point distances only.
For a taxi journey we take the distance as the crow flies between the start and end location of the taxi journey. Once we have this distance we multiply it by 1.03 to take account of average congestion with an average amount of urban driving and we assume the taxi's miles per gallon are the same as those for a RAC medium sized diesel engine.
Over time we aim to refine the accuracy of these estimates. To do this we will need to gather more detailed information about the sorts of vehicles used by transport operators on a journey. For example we may distinguish between diesel and electric trains and how fast the average speed of the train will be on the journey. For buses we may be able to take account of the different numbers of passengers for different sorts of buses to reflect the difference between a typical London bus, a typical rural bus and a typical urban bus outside London.
12.4) What assumptions do you make in comparing CO2 emissions for car journeys with public transport journeys?
We try to give you an overall feel for what the difference in CO2 emissions produced per traveller would be.
If the journey you have chosen is by car, we calculate the emissions for that journey following the actual route to be taken, if you then select a public transport comparison we estimate the emissions based on travelling the same straight line distance as the car using the different types of public transport. This is for comparison only and there may or may not be viable public transport links between the original car journey locations. If you want to actually travel by public transport use the appropriate planer to find your travel options.
Similarly if we have planned you an actual public transport journey, when you ask for a comparison we estimate both car and other types of public transport emissions based on travelling the same straight line distance as the selected public transport journey. For the car we estimate how much fuel would be used and hence emissions generated. In this case we assume you would be travelling in a medium sized petrol car travelling on a combination of urban and interurban roads, with average congestion (resulting in the emissions being increased by 1.03 compared with the unrestricted values).
We compare the predicted CO2 emissions produced for a car journey with a specified number of occupants with an estimate of the total CO2 emissions that would be produced for each traveller on one or more types of public transport.
However, a car and public transport are different. In absolute terms of reducing CO2 it is always better to use a scheduled public transport service. This is because when you use a car the result is an extra journey is made and extra CO2 is emitted into the atmosphere. On the other hand, when you use public transport journey by bus, coach, plane or train the journey is always scheduled and would take place whether or not you travelled. So in real terms there is no additional CO2 emitted.
12.5) What assumptions do you make about bus and coach journeys?
To determine whether emissions for either a bus or a coach are shown we first estimate the journey distance.
We assume you will be travelling by bus on short journeys of less than 30KM. So on short journeys we only show the predicted CO2 emissions for a bus.
On the other hand, if we estimate that the journey distance is longer than 30KM we assume you will be travelling by coach. So on longer journeys we only show the CO2 predicted emissions for a coach.
To calculate bus emissions we use a factor based on the average bus miles per gallon and number of passengers carried and to calculate coach emissions we use a factor based on the average coach miles per gallon and number of passengers carried.
12.6) Why do you use the same colours as the Energy Efficiency Ratings?
For a number of years it has been widely recognised that electrical goods such as washing machines have been supplied with a standard efficiency rating and this has been presented on a standard A-G chart, from green to red. This chart's usage has now been widened and is recommended for use by the Vehicle Certification Agency (VCA) for all new cars. Transport Direct assumes that cars with one occupant and low emissions are likely to be in the green and yellow area of the bar and cars with one occupant and high emissions will be in the orange and red area. This “rating” will remain constant irrespective of the distance of the journey, but the amount of emissions generated will increase as the journey distance increases.