4Rationale#

There are, essentially, two schools of thought on the display of messages on VMS:

• Always display a message on a sign regardless of whether or not a situation exists on the carriageway which requires a response/action by motorists (lane closed due to roadworks/incident, object on carriageway etc.).
• Only display a message when unusual conditions exist.

The primary argument in support of the first approach is the need to maintain motorists’ confidence that the signs are functional and not faulty. A secondary argument is that motorists are conscious of the cost of the signs and expect to see some benefits from the investment in the asset.

Support for the second philosophy stems primarily from the human factor principles that motorists should not be given trivial information or information that they already know and that the communication system will be more effective by limiting the display of information to situations when some response by motorists is required. These human factor considerations are particularly important in those environments in which there are a lot of other competing demands for motorists’ attention or where traffic volumes are high (greater than 85% of capacity).

The human brain absorbs a large amount of information whilst driving. In these situations the driver is required to filter out less relevant information. The risk is that frequent exposure to trivial information will ultimately result in the messages not being registered at all. In addition, unnecessary or unexpected distractions in high traffic volumes can have a significant impact on driver following behaviour. As the traffic flow approaches capacity, traffic conditions become very unstable and considerable fluctuations in both speed and concentration can occur in response to distractions. In less demanding driving situations, the converse argument has also been presented - where there is little other stimuli the messages will continue to register, irrespective of how trivial they might be.

This policy reflects a balance between both message composition philosophies, due to the vastly different road environments in which VMS are installed in Queensland. Supplementing traffic messages with filler messages that are directly relevant to the driving task, likely to enhance the safety or performance of State-controlled roads or influence or inform the public in cases of potential or declared natural disasters will maintain the effectiveness of the signs and ensure that there is a reasonable probability that motorists encounter a message.