Author Topic: I have been given 24-hour traffic flows. How do I apply them?  (Read 6037 times)

roger

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You have been given the total traffic flow for each 24-hour period, in your case the 24-hour AADT (annual average daily traffic flow).  You haven't been given the traffic flow for each hour of the 24-hour day.

So in NoiseMap, you do not enter this as a 24-hour traffic flow. [There isn't any box for entering this value.]
Furthermore, do not divide the flow by 24 and enter this value for each hour of the day.  This is because the flow varies over the day, being greatest in peak periods (rush hours) and less at night.  This is called the diurnal variation. If you do know the diurnal variation, you could factor the 24-hour flow into 24 one-hour values, but most likely you do not have this information.

The usual approach is to convert the 24-hour AADT into an 18-hour AAWT (annual average weekday flow) as this is the value used by the Noise Insulation Regulations (to err on the high side, as weekday flows are usually higher than weekend flows).  You can then enter this value into the 18-hour flow column of the Traffic Flow Manager.

CRTN has a built-in presumption of the diurnal flow variation, based on the 18-hour AAWT flow, and the noise levels are calculated on the basis of that presumption.

So how do you convert 24-hour AADT to 18-hour AAWT?  There is now a lot of traffic data published on various government and local authority web-sites.  Some of these give traffic counts over various time periods, often for specific roads. From these, it is possible to deduce a conversion factor.  For example, on a certain part of the M25, the 18-hour AAWT flow is 95 % of the 24-hour AADT flow. Also, the 18-hour AAWT percentage of heavy vehilces is 94 % of the 24-hour AADT (there is a slightly higher proportion of heavy traffic at night).
These values are typical of motorways.

You could also use this source of traffic data to derive typical conversions of 24-hour to 24x1-hour values, but this generates a lot of data and is unlikely to greatly improve calculation accuracy except where the diurnal variation is atypical - for example, close to a port or distribution centre.

roger

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Re: I have been given 24-hour traffic flows. How do I apply them?
« Reply #1 on: February 19, 2018, 02:29:20 PM »
Please note that recent investigations have shown a problem with using TRL/Defra Method 1 with 24 x 1-hr flows where the flows are low. Please read the advice under the Support forum above.