3Speed parameters for temporary roads#

AGRD03 does not provide any specific guidance for the design of temporary works but describes a range of conditions which may be appropriate for the design of temporary works. It identifies that:

Temporary road design has historically and typically been based on application of a design speed 10 km/h greater than the nominated temporary posted speed given the lack of available speed data for the temporary alignment. This practice then provides a forgiving environment but perversely often results in an alignment that does not encourage self-compliance with the speed limit due to the apparent higher standard of the temporary road.

It is therefore a critical element in the consideration of any design exception that the design speed must align with the operating speed and cannot just be assumed as artificially low. Where design exceptions are included based on a design speed, appropriate mechanisms must be in place to ensure that there is a high level of speed compliance typically due to a supporting road environment. Where temporary speed limits are posted at speeds significantly lower than the permanent limit, there is a need to create a geometric environment which encourages lower speeds.

A key relevant issue is the selection of the design speed to be used in the design of all other parameters. It is however noted that the design speed should be selected on the basis of the speed at which traffic could be reasonably be expected to operate given the road design, alignment and roadside environment. It is noted that the desired speed for a section of road is influenced by the following elements:

Roadside environment – topography in rural areas, development density and type (i.e. built environments) in urban areas.
Road characteristics – geometric standard (predominately horizontal alignment; to a lesser extent, vertical alignment; and lane widths), frequency of intersections and accesses, sight distance, parking provisions etc.
Speed limit.
Road function – to the extent that on important roads such as motorways and highways, drivers are less willing to accept reductions in desired speed.

As a case example, the Ipswich Motorway upgrade – Rocklea to Darra (2020) included a temporary road alignment which included an aesthetically unpleasing alignment, very obviously within a worksite, and narrow offsets to the temporary road safety barriers. In this section of the works, operating speeds appeared to be close to the posted 80 km/h limits with high levels of driver compliance.

In other cases, it has been observed that a level of traffic congestion, greater than normal congestion levels, during the works also aids in improving compliance with posted speed limits. Customer requirements such as acceptable Level of Service for operation should be determined within the Traffic Management Plan with a consideration of the impacts on speed limit compliance.

Hence selection of a design speed which appears artificially low to drivers and is likely to result in low levels of speed compliance, would subsequently reduce the support for isolated reductions of the road design parameters for a temporary road. Design speed selection therefore should be based on the desired and operating speed for the temporary road, not based on a low posted speed that has low levels of self-compliance. Where design exceptions are introduced into the road design for the temporary road, the actual speeds of traffic should be monitored and measures taken to ensure suitable levels of speed limit compliance.

AGRD03 describes parameters around the operating speed for temporary works and notes that where the temporary road is in close proximity to construction works it is usually desirable and necessary for the operating speeds on the temporary road to be less than the operating speed on the approach section. The addition of visible cues will alert the driver to the need for reduced operating speed as well as the speed zoning. This may be achieved by a combination of:

Signage, safety barriers and anti-gawking screens that reinforce the presence of road works.
Narrower cross-section elements where practical to give the appearance of a lower standard of road.
Different pavement surfacing appearance and/or type.
Active traffic management, including temporary traffic signals and stop/go traffic control.

Several project managers on the Department’s projects reported a desire to select lower design speeds. However, it was often also noted that compliance with posted speed limits was low and that the requirements for temporary alignments do not encourage self-compliance with the posted limits.

It is re-emphasised that any application of the design exception opportunities discussed in this technical note should only be undertaken in coordination with the necessary measures to ensure a high level of driver compliance with the temporary posted speed limits at the roadworks project.