4.3Coordination of horizontal and vertical alignment#

Horizontal and vertical design parameters for roads include a set of criteria with regards to the coordination of horizontal and vertical alignment. For temporary roads this can have significant impacts on issues associated with the alignment of the road. This in turn can have a significant impact on both the costs of the construction of the temporary road and on the constructability of the works in constrained sites.

4.3.1National / International guidance – literature review#

The following guidance relating to horizontal and vertical coordination may be applicable to temporary road alignments. Austroads AGRD03 details three overall considerations associated with the coordination of the horizontal and vertical alignment being:

Safety – in particular it is noted that a major safety risk is associated with the sight distance to a horizontal feature that the driver needs to react to (curves, cross section changes, start of median etc) which can be obscured if it is located within a vertical curve.
Aesthetic – design features which create the most pleasing appearance.
Drainage – to ensure water flow paths do not result in excessive water depths within the road alignment.

There is no existing guidance detailed in State / Territory or international road design guidelines relating to horizontal and vertical coordination for temporary roads. However, in considering potential design exception criteria for horizontal and vertical coordination for temporary roads the Department considers the following to be appropriate for consideration with regards to the three overall considerations for horizontal and vertical alignment.

4.3.1.1Safety considerations#

For temporary road alignments, design considerations relating to safe operation of the road remain applicable. This design requirement primarily relates to the visibility of horizontal alignment features which may be potentially hidden by a vertical alignment feature such as a curve. The primary mechanism by which safety is achieved is by providing sufficient visibility to the line marking or kerb line for the change of alignment, cross section or introduction of median islands.

4.3.1.2Aesthetic considerations#

For permanent road alignments, aesthetic considerations relating to combined horizontal and vertical alignment are predominantly focussed on providing a pleasing appearance to the road alignment and limiting sudden changes in lateral and vertical acceleration experienced by vehicle occupants. For temporary road alignments, these may not need to be considered. In fact, the lack of a pleasing appearance may assist in ensuring that drivers remain alert to the changed traffic conditions and may play a role in encouraging higher speed limit compliance.

4.3.1.3Drainage considerations#

For temporary road alignments, drainage considerations relating to horizontal and vertical are predominantly focussed on water flow paths across the alignment. These considerations should continue to be observed but may take into account factors and mitigating measures such as:

Duration of the works and the likelihood of wet weather during the works.
Variable message signs to alert drivers to the presence of wet pavement and water on road hazards ahead.
Crash records and near incidents – should be assessed regularly to establish if a lower speed limit needs to be implemented with additional design measures introduced to address vehicle speeds should crash records be unacceptable.
Ongoing monitoring – to identify emergent safety issues associated with the lack of horizontal and vertical coordination.

4.3.2Case Examples#

No case studies were identified where design exceptions for the coordination of horizontal and vertical alignment were implemented.

4.3.3Design Exception opportunities – horizontal and vertical alignment coordination#

Horizontal and vertical alignment considerations should continue to be applied for safety and drainage considerations with some of the mitigating measures identified below potentially allowing alternative means of achieving these coordination requirements. Aesthetic considerations for horizontal and vertical alignment may not need to be observed for temporary road alignments. Any design exception would require consideration of the following safety and drainage considerations:

Speed limit compliance – to ensure that the majority of traffic is travelling at or below the posted speed limit.
Lighting of the alignment – to ensure that drivers can see objects ahead including devices used to delineate changes in alignment.
Crash records and near incidents – should be assessed regularly to establish if a lower speed limit needs to be implemented with additional design measures introduced to address vehicle speeds should crash records be unacceptable.
Duration of the works and the likelihood of wet weather during the works.

4.3.4Mitigating measures – horizontal and vertical alignment coordination#

To ensure that safety considerations are addressed, for temporary road alignments there may be an opportunity to use the following to highlight changes in alignment or potential hazards relating to water flow across the carriageway:

Alignment markers placed along barriers or delineating devices.
Use of illuminated arrow boards or variable message signs to advise of alignment changes.
Delineation devices such as bollards and temporary road safety barriers.
Variable message signs to alert drivers to the presence of wet pavement and water on road hazards ahead.
Ongoing monitoring – to identify emergent safety issues associated with the lack of horizontal and vertical coordination.