Speed limit signs in Australia are erected in accordance with national standards. In Western Australia, the agency responsible for determining the speed limit is Main Roads WA (MRWA). The speed limit in any particular area is determined by engineers and based on a number of factors including the general topography of the location, the urban density and road design.
The Road Safety Commission is responsible for research and policy which develops road safety strategies.
Apart from the commencement and end of a speed zone signs, MRWA erects 'repeater' or 'additional' speed signs further along the route within the zone. That authority endeavours to space them as follows and some variances do occur, and distances are not bound by legislation:
- 40 km/h zones apply in areas with high levels of pedestrian activity, such as shopping centres
- on some roads outside schools, permanent 40 km/h speed limits are in place
- time-based speed limits outside schools apply as displayed on the speed signs
- the default speed limit is 50 km/h
- speed signs are not displayed on all roads in built-up areas. In these cases, the default speed limit is always in effect
- speed limits on many arterial roads are posted by the appropriate speed limit signs and can range from 60 km/h to 90 km/h
- the default speed limit for freeways is 100 km/h unless zoned otherwise
- variable electronic speed limit signs also operate throughout Western Australia
- the default speed limit for open areas is 110 km/h. This applies on roads outside built-up areas
- the derestricted speed zone limit is 110 km/h and this applies only to areas as signposted outside the metropolitan area.