Traffic noise is typically due to heavy vehicles (e.g., trucks, haulers, tractors, tankers, trolleys, buses, etc.), light vehicles (e.g., cars, vans, minivans, pickups, etc.), motorcycles and special vehicles having additional noise sources (e.g., ambulances, fire trucks, police cars, etc.).
This kind of noise is directly originated by multiple components: engine and transmission, rolling tires over the asphalt, aerodynamic, braking systems, additional engines (e.g., radiators, air-conditioners, etc.) and other vehicle-mounted devices (e.g., horns, sirens, bells, whistles, etc.).
In real scenarios, noise levels are given by a mix of heterogeneous traffic conditions: different speed of vehicles, mix of different vehicles types, different vehicle age, etc.
Other variables should also be taken into account, as they contribute to the overall roadway noise levels by their interaction with the vehicles: road surface types (different types of asphalts produce noise levels that can differ up to ±8 dB), roadway geometrics, surrounding terrain and buildings (acoustic waves interact with obstacles according to a series of propagation mechanisms), local weather conditions (wind and temperature gradients can refract waves).
Vehicle speeds affect significantly noise levels: at extremely low speeds, braking and acceleration events are the most significant noise source, whilst at low speeds engines represent the main source of noise. As the vehicle speed increases (beyond 60 km/h), the tires rolling over the asphalt become the loudest source. At very high speed (beyond 150 km/h), aerodynamic noise prevails. The noise from heavy trucks, instead, is mainly due to their engine, both at low and high speeds, since these engines are larger and placed in higher position if compared to other vehicles.
From an acoustic modeling point of view, single vehicles and vehicle flows have to be considered differently. On the one hand, a single and isolated vehicle can be modeled as a point source placed above the road, whose radiated sound energy is shaped as a semisphere centered on the point source, thus the noise decrease is a function of the square of the distance from the source. On the other hand, a flow of vehicles can be represented by a linear source, placed above the road, whose radiated sound energy is shaped as a semi-cylinder coaxial to the linear source, thus the noise decrease is a function of the distance from the source.
As a reference, we can consider the following average values, at 15m from the observer, for speeds not exceeding 50km/h:
- Mixed traffic flow: 85 dB
- Single autobus: 90 dB
- Single heavy truck: 90 dB
- Single van: 80 dB
- Single motorcycle: 85 dB
- Single car: 75 dB
- Single ambulance with siren: 105 dB