2.2Background#
A variable speed limit zone may be applied on a motorway, long bridge or in a tunnel to allow a reduction in the posted speed limit at times when road safety and performance are compromised, where full time lower static speed limits are inappropriate. The variable speed limit zone is implemented through the use of variable speed limit signs and selected static signs.
Variable speed limits and lane control signs are integrated into a single set of signs as shown in Figure 2.2 where:
- the lane is open to all traffic at the indicated posted speed limit when a variable speed limit is displayed above the lane.
- the lane is soon to close to all traffic when a flashing red cross is displayed above the lane, and drivers must leave the marked lane as soon as it is safe to do so.
- the lane is closed to all traffic when a red cross is displayed above the lane and drivers must not drive in the marked lane past the signal, and
- a driver may enter a closed lane to exit the motorway if a diagonal up arrow (exit arrow) is displayed.
These signs are installed to increase efficiency and safety during times of congestion, incidents, inclement weather and planned events. These signs, when installed permanently, can be used to aid the control of traffic through work areas for maintenance or construction.
The variable speed limit and lane control signs are connected to a central software control system (STREAMS or similar) which allows operators to manage the operation of the signs in accordance with the MUTCD and other principles.
It should be noted that the spacing of signs along the mainline will vary according to the location of entrance and exit ramps, and in accordance with the Austroads Guide to Traffic Management and relevant supplements. Variable speed limit signs installed on motorways with more than three lanes are mounted overhead and integrated with lane control signs (Figure 2.2).
Default static speed limit signs TC1568 are installed at entrance points to a variable speed limit zone and at changes of the default speed limit along the mainline carriageway.