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Calculation of Road Traffic Noise
Technical Description
Calculation of Road Traffic Noise is the statutory method for calculating noise from roads in the UK.  It was originally published in 1975 and was updated in 1988.  It is based on research that dates back to the 1960s and is extremely well proven with good accuracy.  However, it calculates noise levels in terms of the L10 index.  Whilst this unit correlates well with human response to road traffic noise, it has been largely superseded by the Equivalent Continuous Sound Level (LAeq).  Fortunately, at locations alongside roads, these two indexes are highly correlated, which means that it is possible to predict the Laeq level from the LA10 level with a high level of certainty.

The UK government has published a procedure that allows LA10 (18-hour) levels to be converted into Lden and Lnight values.  Because motorways carry a higher proportion of traffic at night compared with other roads, the conversion factors for motorways are different from the conversion factors for other roads.

Calculation of Road Traffic noise involves several steps.  The first is to divide the road network into as many segments as necessary to ensure that there is uniform traffic and propagation conditions for each segment.  In a large city, this could mean tens of thousands of road segments.  The calculation method is then applied to each of these segments.  The first step is to calculate the noise generated by the traffic on the segment at a notional reference location which is 10 m from the edge of the nearside carriageway and at a height of 1.2 m above it.  This is called the Basic Noise Level.  The Basic Noise Level is then adjusted to account for the distance of the receiver point from the road, (geometric spreading of the sound energy), the height of the receiver point above the road and the type of ground cover (ground effect, sometimes called ‘ground absorption’), the effect of any intervening noise barriers and the effect of reflecting surfaces.

The calculation of the effect of barriers is particularly complex, as there can be many barriers between the road and the receiver, particularly in a city with many buildings. 

The contribution from each segment is also adjusted according to the angle of view that it subtends at the receiver. 

The contribution from all the road segments is then added to obtain the total noise level at the receiver.


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