# Basic Equations

## L 10 Levels

For road schemes consisting of more than one segment, the predicted level at the reception point is to be calculated by combining the basic hourly or 18-hour levels predicted for *N* segments, L_{10} „ using

*Figure 12.4.* An illustration of shortest slant distance between a reception point and an effective source line representing a flow of traffic (Figure I from CRTN).

where

The following corrections are required for each segment:

- -
**A**_{pV }*i*correction for mean traffic speed - - A
_{gj}correction for road gradient - - Д
correction for road surface texture_{TDJ} - - Д
_{0}, correction for distance - - Д
_{С}с,/ correction for ground cover - -
*A,*barrier correction for - - Д
_{0}я,, correction for opposite facade - - Д
correction for size of segment._{S i}

The basic А-weighted hourly noise level hourly is predicted at 10 m from the nearside carriageway according to the following equation

where *q* is the hourly traffic flow (in vehicles/hour).

Alternatively, the basic А-weighted noise level in terms of total 18-hour flow is

where Q is the 18-hour flow.

Here, it is assumed that the basic velocity V is 75 km/h, the percentage of heavy vehicles *p* is 0 and the gradient G is 0%. It is also assumed that the source line is 3.5 m from the nearside edge of the road for carriageways separated by less than 5.0 m. If the two carriageways are separated by more than 5.0 m, or where the heights of the outer edges of the two carriageways differ by more than 1.0 m, the noise level produced by each of the two carriageways must be evaluated separately and then combined using expression (12.15). For the far carriageway, the source line is assumed to be 3.5 m in from the far kerb and the effective edge of the carriageway used in the distance correction is 3.5 m nearer than this, i.e. 7.0 m in from the edge of the far-side carriageways.

## Corrections for Mean Traffic Speed, Percentage of Heavy Vehicles and Gradient

The correction for percentage of heavy vehicles and traffic speed is determined using the following expressions:

т , ,, .... 100*f* 100*F*

In this expression, the percentage of heavy vehicles is *p =*-*- =*-,

<7 Q

where *f* and *F* are the hourly and 18-hour flows of heavy vehicles, respectively. Equation (12.19) is applied to the basic hourly or 18-hour levels. The value of V to be used in (12.19) depends upon whether the road is level or on a gradient.

For level roads, the traffic speed, V, to be used in the calculation (12.19) is the value for the appropriate class of road. For roads with a gradient, traffic speeds must be reduced by Д V km/h evaluated according to the gradient and the percentage of heavy vehicles from

where G is the gradient expressed as a percentage. Once the speed of traffic is known, then the adjustment to the А-weighted level for the extra noise from traffic on a gradient is calculated from