CRTN states that its formula are valid over a 'quoted range of validity', although the range is implied by the charts rather than explicitly stated. It's easy to forget this range of validity when using computer software. NoiseMap attempts to trap out-of-range values and to issue warnings. For traffic flow data, the range of validity implied by the charts is:
Flow Rate - 50 to 10000 vehicles per hour and 1000 to 200000 vehicles per 18 hours.
Mean speed - 20 to 130 km/h
Percentage of heavies - 0 to 80 %
However, if you use TRL Method 2 (i.e. using day, evening and night traffic flows to calculate Lden values), you MUST have non-zero values for flow rate, speed and percentage heavies for the three time periods and the 18-hour value, otherwise the method cannot be used.
The explanation is as follows:
The method has a term in the conversion formula that takes 10 times the log of ratio of the product of the period flow data values in the numerator and the 18-hour flow values in the denominator. So if the percentage of heavies is zero in the 18-hour case (and thus zero in the period case), the conversion term is zero divided by zero and the value can't be calculated. The term would be uncertain if any of the periods have zero values, as it would become the log of zero which is also undefined. [A more normal expectation is that the flow will be, say, 20 % of the 18-hour flow, with the speed and percentage heavies not greatly different from the 18-hour values. This would give a value of -7 dB.]
Thus method 2 cannot be used if the percentage of heavies, flow rate or speed are zero in any period. You might be able tp fix this by putting in a small value, say percentage heavies = 0.1, which causes this term to tend to a value of zero, and it drops out of the equation.
For the other values, remember the limits on validity of speed and flow rate. If you put a speed of less than 20 km/h (or the gradient correction causes it to fall below this value) NoiseMap uses a value of 20 km/h.
If the flow rate is less than 200 veh/h or 4000 veh/18 h, (common on residential roads and non-motorway roads at night) then CRTN makes a low-flow correction to L10, but the conversion formulae do not adjust the period Leq for this effect. It is, however, implied in the night-time conversion formula for non-motorway roads, according to the TRL report.
