The total external noise cost for Brussels is the monetary value per dB, multiplied by
the noise level above the threshold of 50 dB(A), multiplied by the number of road km
where a noise externality is generated (we assumed 500 km supra)?.
To compute the resulting marginal external noise cost (MENC) in the reference equi-
librium, we derive the total external noise cost function with respect to the number of vehicle km. Using the noise formula (8) given above, we find that the MENC of a car
km is given by*: