TRL report PR/SE/451/02 (2002) sets out three methods of obtaining the Ld,e,n indicators from CRTN methodology.
Method 1 is the preferred method but requires you to have the 24x1-hour flows on the road. You also need to know if the road is a motorway or not, especially when flows are below 200 v/h at night. With this method, you calculate each of the 1-hour Leqs and then sum them for the required periods, applying the appropriate penalty for evening and night levels.
Method 2 can be used where you have the traffic flows for the day, evening, night and 18-hour periods. (Note that the 18-hour period is from 0600-2400, whereas the day period is 0700-1900 and the evening period is 1900-2300 and night is 2300-0700. This means that the 18-hour period cannot be got from the day and evening periods.)
Method 3 uses only the 18-hour flow. However, the conversion is different for motorway and non-motorway roads.
There is a further issue. The recommendation in the 2002 report took into account that the road network is divided into acoustically homogenous segments as advised in CRTN. Clearly the result should not depend on how the road network is subdivided. However some software (NOT NoiseMap!) divides the segments into smaller sections than advised in CRTN, resulting in lower noise levels from each short segment, and thus affecting the conversion to Leq. DEFRA therefore changed the procedure to require the segment noise levels to be summed before conversion to Leq rather than afterwards. This is the difference between the 2003 and 2005 conversion methods in NoiseMap.
Which method is more accurate? the TRL report has a number of graphs which show the scatter of results. In practice, Method 3 seems acceptable for most cases, but where the night-time traffic flows do not fit the usual pattern, then one of the other methods should be better. However, this assumes that you have accurate counts of flow, percentage of heavy vehicles and speed. These are often very difficult to obtain. But if you are dealing with a 'managed motorway' there is likely to be a detailed traffic flow model that can provide these for you.
Finally, note that NoiseMap takes care of the complexities of the calculation: all you have to do is to set the appropriate Traffic Flow Type in the traffic manager and to enter the required flow data.
