7.5.4 En Route Information
Variable message signs
Variable message signs (VMS) are widely used to provide road users with information about road and traffic conditions. VMS messages are electronically generated and can be changed to display predefined or free text information, figures or symbols. They have the ability to display a large range of individual messages for the purpose of directing, warning or guiding road users. More detailed information on VMS and their underlying technologies can be found in Part 10 of the Guide to Traffic Management (Austroads 2016a).
This section gives guidance on the use of VMS for the provision of traffic information. VMS technology can also be used for traffic control purposes; examples include lane control signals and variable speed limit signs. Other applications of VMS are discussed in Part 10 of the Guide to Traffic Management (Austroads 2016a).
VMS may be used as individual signs to address a particular issue, or as part of a system to manage traffic along a road or within an area. A common systemic use is for the management of traffic during incidents on high volume urban motorways or on the arterial road network. The need for VMS is typically determined on a case-by-case need of driver information on a route. This will be influenced by factors such as:
- availability, utilisation and driver understanding of alternative routes
- congestion influences on a subject route
- traffic characteristics on a subject route
- need to provide specific driver information (e.g. travel time) or support other management tools (e.g. identify reason for VSL speed reduction).
A VMS may be a permanent sign or a temporary sign mounted on a trailer or vehicle to meet a short-term need.
Whatever the use of a VMS the same issues of sign conspicuity, legibility and comprehension as for static signs, compounded by the VMS capability of spreading a message across more than one screen (or frame), must be addressed.
Guidelines to achieve the required levels of conspicuity, legibility and comprehension are given in Part 10 of the Guide to Traffic Management (Austroads 2016a) on the following:
- sign dimensions and other physical characteristics
- types of messages and symbols
- message content and format
- message load and exposure time
- location and spacing.
The guidelines include recommended (Austroads 2016a):
- message statements, comprising
- problem statements
- location statements
- effect statements
- attention statements
- action statements
- time and date statements
- generic message sets.
VMS operation can be manual, semi-automatic or fully automatic.
Manual operation relies primarily on a pre-prepared set of generic messages; free text may also be used.
Most existing systems are manually operated, the operator being aided by computer outputs such as travel times on the competing routes, or simply by other data from the field (such as video images), or by pre-determined strategies designed off-line for a given number of situations. These systems cannot generally handle complex situations such as incidents in a grid motorway network, or multiple incident situations, and are often unable to give an updated strategy, once the first one is implemented. They also generally do not include forecasts (of the demand and of the impact of the current strategy), so they are far from producing an ‘optimal’ strategy, and they can even produce situations worse than a ‘do-nothing’ strategy (due to risk of creating over-diversion).
Semi-automatic operation utilises lane occupancy detectors or closed-circuit television (CCTV) cameras that may be linked to alarms to provide warning of an incident and to recommend specific messages for implementation by a traffic management centre operator.
An example of semi-automatic operation is the wide area algorithmic response (WAAR) application implemented in New South Wales. This offers the operator an incident response somewhere between free text manual settings of VMS (slow to implement across many VMS) and the very targeted VMS settings contained within a pre-defined strategic plan.
When the operator declares an incident on the network, WAAR ‘walks’ upstream from that incident location (on all possible paths) and identifies VMS which could be used to advise of the incident. The distance it walks is configurable but also influenced by the incident type/severity (i.e. it looks further afield for the bigger incidents). Once all VMS have been identified, the application then groups the VMS according to size and offers the operator a selection of fairly generic messages for these VMS groups. These can then be simply and quickly set en masse as the first part of the response. As more specifics about the incident are made available to the operators, they can then supplement the WAAR with a plan and/or some more specific messages on selected VMS.
In Europe some systems, such as MOLA (UK) or VISUM-online (Germany), are able to simulate the impact of a set of possible strategies designed off-line (generally two strategies) in order to help the operator to choose the best one.
Fully automated systems require the integration of incident detection systems with message selection and deployment for the management of incidents, congestion and the impact of adverse weather conditions. For a large network they can be time consuming to develop and maintain across all possible incident locations, covering all contingencies.
One system, OPERA (France), automatically generated guidance strategies using forecasts and a real-time expert system, thereby adjusting itself to current traffic patterns and their forecasts. Benefits of up to a 50% reduction in time wasted due to incident occurrence were reported (World Road Association 2003).
From an operational point of view, and because VMS can be used for different purposes, it is important to establish priorities for their use. For example the following is a suggested descending order of priority based on road safety and traffic flow considerations (Queensland Department of Main Roads 2009):
- weather related conditions
- parking guidance
- special events
- road safety messages
- community benefit messages
- general transportation messages.
The last three applications can be considered as filler or standby messages, which may only be displayed if considered useful and appropriate by the relevant jurisdiction. Special events can fall into this category or may be an incident in their own right. Road agency VMS installations should not be used for commercial advertising under any circumstances. It is important that the highest relevant priority message be displayed. This requires the ongoing monitoring of conditions and incidents. If a high priority message would be appropriate, it will bring VMS into disrepute if a low priority message is displayed. Under these circumstances it would be better for no message to be displayed.
The major benefit of VMS is that information can be conveyed to drivers en route in order for them to make more informed decisions on their chosen route and reduce travel time. The VMS system can also be utilised to highlight hazardous conditions and therefore can aid in reducing accidents. Austroads (2003) in its review of the benefits of ITS demonstrated that VMS have varying degrees of effectiveness at diverting traffic from incident routes which can result in varying degrees of travel time savings arising from those incidents. A literature review in Austroads (2007g) reported savings in (average network) travel times ranging from 2% to 4% arising from widespread implementation of VMS.
An automatic VMS system can have a quick response time, from the time of the incident being detected to a message relaying the situation to the driving public being displayed. This can result in possibly reducing further congestion by relaying information to drivers and giving them a chance to divert from their route. A fully automatic VMS system also needs minimal human interaction before implementing the message.
A possible disadvantage of the VMS application is that it may result in a higher diversion rate than the surrounding road network can handle, potentially resulting in congestion on the surrounding road network (Chatterjee et al. 2000). The use of VMS should be carefully considered in the light of the likely impact of diverting traffic onto a possibly lower capacity network. Eves (2005) reported that, depending on the severity of the incident and the time taken to clear the incident scene, traffic operations managers found in hindsight that it was often better not to divert.
Travel time signs
The travel time sign is a particular type of VMS providing travel time estimates directly to motorists on freeways and motorways. A system of these signs has been installed in Melbourne on the Monash, West Gate, Tullamarine and Eastern Freeways (Figure 7.8). It provides accurate, real‑time information on travel times and colour-coded congestion levels along the freeway corridor. The colour-coded congestion levels help drivers quickly appreciate traffic conditions in advance. Commentary 12 describes the underlying detection system and travel time estimation process.
By providing drivers with accurate real-time estimates of travel time, the signs provide an opportunity for drivers to make an informed route choice. The decision whether to divert or not in response to the travel time information is left to the individual drivers, so the risk of over‑diversion discussed above is reduced.
Source: VicRoads (n.d.), ‘Unpublished photograph’.
In-vehicle navigation systems
In-vehicle navigation systems use global positioning systems (GPS) technology along with digital maps to assist drivers to navigate the road network. Compatible navigation systems can overlay the map information with traffic information delivered to the navigation unit by RDS-TMC (Radio Data System – Traffic Message Channel) technology on a broadcast FM sub-carrier. RDS is the communications standard from the European Broadcasting Union for sending small amounts of digital information using conventional FM radio broadcasts. TMC refers to the TMC Alert-C coding standard for traffic information and location, fully described in ISO Standard 14819. The location tables are integrated into the digital maps provided by navigation system suppliers, enabling traffic information to be displayed at its correct location on the maps.
Information delivered to RDS-TMC compatible navigation systems may include roadworks, incidents and unusual congestion.
Depending on the service provided and the individual navigation system, alerts may be presented by voice synthesis, graphics or by automatic re-routing around the incident.
A recently commenced service takes real-time SCATS and STREAMS data from road agencies (under commercial agreement) and uses this in a travel time model to estimate travel times and delays. Congestion alerts and travel time and delay estimates are then delivered to road users via RDS-TMC (Intelematics Australia 2011). Figure 7.9 illustrates the stages of data processing involved. Figure 7.10 shows the in-vehicle display.
Source: Austroads (2007h).
Source: VicRoads (n.d.), ‘Unpublished photograph’.
Traffic information encoded for RDS-TMC can also be formatted in extensible mark-up language (XML) for delivery via internet applications.
Telephone and radio
The telephone and radio delivery channels can also deliver the same traffic information in-vehicle, en route.