The alignment of a road is the controlling factor to the location of noise sources. For example, road alignment may affect the degree to which: the effects on protected premises and facilities can be avoided, remedied or mitigated; the way noise is propagated between the source and receivers; and the nature and level of noise generated.

Factors to be considered when determining the route and the alignment of a new or altered road include the availability of land, the nature and purpose of the new or altered road, and the safety and geometric design Standards.

When the route for a new road is being considered, noise mitigation measures should be explored at the time the road alignment is selected. This may involve optimising the distance between the road and any protected premises and facilities in order to exploit distance attenuation effects or using the topography of the land to shield protected premises and facilities from the noise source (see the noise propagation section below).

The underlying topography affects not only the need for features such as cuttings and retaining walls but also the gradient of a road. This affects the extent to which drivers of vehicles need to vary traffic speed, change gear and use their brakes. Each of these factors affect the engine load, which in turn affects the amount of noise generated. By reducing the gradient of a road the amount of noise associated with vehicle acceleration and deceleration as well as exhaust and engine breaking can be reduced.

The impact of gradient on truck noise is particularly noticeable. For example, trucks tend to use their engine brakes from around -2% gradient, whereas cars will tend to use their engine brakes on steeper gradients of more than -8%. A 5% reduction in road gradient can reduce noise levels by around 1.5 dB.

For altered roads, especially in urban areas, those designing roads should continue using the horizontal alignment to maximise any shielding effects offered by existing natural or man-made features.

The process of determining the geometric design requirements for a new or altered road alignment can also be used to reduce any likely noise effects. This includes using the shielding effects of embankments, cuttings, retaining walls and solid safety barriers. In addition, it includes design speed considerations (see the traffic management section above).