emission level at the reference distance, dB(A); Ni is the number
of vehicles of ith class that passes during the relevant hour; Vi is
the mean speed of ith class of vehicle, m/h; T is the duration to
calculate the equivalent continuous sound level, 1 h; r is the
perpendicular distance from the centre line of the traffic lane to
the receiver, m; r0 is the reference distance, m; a is the ground cover
coefficient, 0 < a < 1; Waðu1;u2Þ is the revised function for a finite
length of road; u1 and u2 are the angles at the observer between
the normal line from the observer to the source and the lines to
the ends of the finite length of road section, rad; DS is noise
attenuation caused by road structure or noise barrier, dB(A).
When a and DS equal to 0, the model is expressed as follows: