First principles assessment
Why Introduce Variable Message Signs?
The Aims of VMS
The aim of using VMS is to provide drivers with mandatory and/or advisory information, at the roadside, relating to situations ahead or in the immediate vicinity. In relation to congestion VMS can assist drivers in selecting appropriate routes avoiding a traffic queue and to reduce drivers’ stress by improved information. VMS are useful in the event of an incident (e.g. road works, road accidents and other incidents), especially where alternative routes exist. To enhance management of the car traffic flow, VMS may be interfaced to traffic monitoring systems. These systems increase costs but also have the benefits of providing drivers with more correct real-time information.
Signs have different benefits depending on the type of message they are designed for. In particular cases VMS can reduce the likelihood of dangerous driver behaviour. A type of VMS that can be feasible along a minor road is one activated by the speed of a driver that approaches them. This type of sign can for example show a single standard message such as ‘SLOW DOWN’ to inform the driver that she is approaching a bend at an excessive speed. The expected impact of such signs is a reduction in the number of violations of speed limits. These sign are normally free-standing from a control centre. VMS can sometimes reduce the number of signs and amount of information, since their content can vary. This is however rarely a dominant aim of VMS as costs of signs are high.
Potential Uses of VMS
Examples of the potential uses of VMS include:
Some of these are discussed in more detail below.
Lane Closures and Recommended Speed by Lane
VMS can be used to provide information about approaching lane closures and to show variable speed limits, either to enable drivers to slow down before reaching a downstream hazard such as slow/stationary traffic) or to improve the general traffic flow by the use of variable speed limits.
VMS can be used to inform drivers of weather conditions such as fog, ice or snow. The information is normally only useful it relates to downstream conditions on the road ahead. However, they can also combine these with corresponding speed advice.
Parking Guidance Information
A parking guidance system needs four elements: a monitoring system; an organising system; a communication system and a display system. The VMS is the display. Monitoring of the car park needs to be done, the information monitored needs to be communicated and relayed/organised before it is ready to display. (See Traffic Advisory Leaflet ITS 4/03 for more information.)
Parking Guidance is covered in more detail elsewhere.
VMS can be used to display the price of variable tolls on roads or bridges or in areas of congestion charging. Connecting the VMS to relevant traffic flow devices can reduce the need for human intervention.
Generic Dangers and Safety Messages
VMS is often used as a mechanism to provide general road safety advice, for example related to ‘drink driving’ or ‘take a break’.
Good VMS must convey useful and temporally correct information in a form that drivers can understand and assimilate quickly. TRL commissioned the investigation of four main aspects of VMS application (<Source to be confirmed by MVA>):
Distrust among drivers can arise if the messages displayed are not always correct. In particular, congested conditions at motorways junctions can quickly change thereby making a ‘congestion’ warning quickly out of date. Cooper and Mitchell (2002) showed that incorrect responses in navigating were greater where VMS displayed messages contradicted the other signs around them. This is due to the confusion that arises when drivers see a static advance direction sign displaying different information to that shown on a Variable Message Sign. In order to help drivers decide which information to trust the VMS may need to qualify what it displays; for example ‘Delay on M8 going west = 1 hour. Take A89 to avoid’ rather than just ‘Take A89 West’.
The benefits of the signs in general are difficult to measure and little firm evidence exists on demand impacts exist. VMS are often used to inform drivers of congestion, incidents ahead and unexpected delays and can as such reduce drivers’ stress. Use of VMS is therefore not likely to reduce the demand for the road network. VMS are normally designed to aid car drivers using the network and make their journey less stressful, shorter, safer and easier. Due to costs, VMS are normally only used where the information can be seen by many drivers, making them more economically viable on congested trunk roads where alternative routes are available. On the other hand, no firm evidence of that VMS increase demand for car trips exists either. It is obvious that impact on demand highly depends on the type of signs installed and the information provided. Signs can be particularly beneficial for demand management where drivers can be informed of alternative routes or park and ride sites to avoid further delays, but this may require the VMS to be an integral part of a wider and more costly traffic monitoring system (including parking availability at Park and Ride site).
Application of parking guidance and information systems (PGI) can reduce search traffic where parking demand outstrips local supply by directing drivers to alternative sites. In city centres providing better information about parking available on route can be much appreciated by car drivers and reduce travel time significantly at peak times. At the same time these systems may reduce the competitiveness of public transport by eliminating the apprehension and stress some car owners can anticipate for a trip to a city centre, making them use public transport instead. See Parking Guidance Information Systems for more information.
VMS is likely to be more effective in handling demand if the messages are available to the public before they begin their journey. In New Jersey there is a direct link from the VMS sign to a page on the New Jersey Turnpike Authority website so those with internet access can see whether there are any roadworks or delays on the route they plan to take. In this scheme, the revenue generated from the tolls will pay for the VMS since a turnpike is a road which has tolls on.
VMS may be used to display the roadside air quality at various points across a city, supplemented with public information displaying air quality targets. A benefit of such a message is that those that drive the most are the most exposed to the message. However, it is suggested here that such information has no direct impact on car usage, unless the information is more specific and provides information on travel alternatives, such as public transport route options and travel times.
The table below shows how VMS can affect the vehicle kilometres travelled by car.
There will be no significant change in demand response of VMS as time goes on as far as the technology of VMS will not be improved to provide more beneficial and effective information.
Impact on vehicle kilometres travelled will be largely dependant on type of VMS installed and the design of the integral Intelligent Transport System. Increased positive impact on vehicle kilometres of route changes can be expected if successively improved travel time information can be provided at more sites concerning Park and Ride (e.g. availability of parking, costs, route frequency, next suitable departure).
There will physically be no increase in the supply of road space by VMS, but providing effective information may make more-efficient use of existing road network capacity where alternative routes with peak time capacity exists.
The cost of signs is significant. Up to date central and local government spending make up a large proportion of the money required.
The costs of VMS are made up of purchase, operating and maintenance.
The prices of VMS depend on the types of technology and vary significantly
depending of type of sign and the appropriate data and sensors required
to compute an appropriate message to drivers. For a list of manufacturers
and suppliers see The Association for
Road Traffic Safety and Management. Approximate capital costs are
£500,000 for 12 VMS at 1994 prices in London. Comparatively the cost of
a VMS will be significant greater than the cost of a regular sign giving
a single message, but generally they also bring additional functionality.
In some places the cost of this will be less than the cost of the time
people waste for example if going onto a closed section of road. There
might therefore be scope for some private investments.
Expected Impact on Key Policy Objectives
Signs have different benefits depending on the type of message they are designed for. The benefits of the signs in general are difficult to measure. The scale of contribution depends in several aspects highly on impact on car traffic demand and which types and for what purposes VMS are implemented and the design of these systems.
VMS are often used to inform drivers of congestion, incidents ahead and unexpected delays and can as such reduce drivers' stress. Signs can be particularly beneficial where drivers can be informed of alternative routes or park and ride sites to avoid further delays, but this may require the VMS to be an integral part of a wider and more costly traffic monitoring system. It is not yet known to what extent suitable alternative routes actually exists where VMS are put up.
Clear facts seem to exist that VMS are not likely to distract drivers if designed properly. A reduction in the number of violations of speed limits can be expected where 'SLOW DOWN' signs are put up. While as yet there is little firm evidence of the safety benefits of VMS in general.
VMS can be used to improve car traffic flows and drivers' safety. Cooper and Mitchell's (2002) final report on the Safety and Effectiveness of the wider use of VMS found that:
Cooper and Mitchell also reported that the effect of different types of VMS message affect the speed and accuracy of navigational decisions:
Chatterjee et al (1999) completed a study of the effectiveness of Variable Message Signs in London. The purpose of the signs is to notify motorists of planned events and current network problems. To guide investment and operational decisions an understanding is required of the impacts of VMS information. The study employed stated intention questionnaires to investigate the effect of different messages on route choice. Logistic regression models were developed to relate the probability of route diversion to driver, journey and message characteristics. The models indicated that the location of the incident and the message content are important factors influencing the probability of diversion. A survey of drivers' actual responses to a message activation showed that only one third of drivers saw the information presented to them and few of these drivers diverted, although they found the information useful.
Boyle and Mannering (1998) in their report titled, 'Impact of traveller advisory systems on driving speed: some new evidence' used a full sized fixed base driver simulator to collect data on drivers speed behaviour under four different advisory information conditions in-vehicle messages: in vehicle messages, out of vehicle messages, both in and out of vehicle messages and no messages at all. The findings of this study suggest an interesting phenomenon in that, while messages are significant in reducing speeds, drivers tend to compensate for this speed reduction by increasing speeds downstream when such adverse conditions do not exist. As a result the net safety effects of such message systems are ambiguous.
Expected impact on problems
Expected winners and losers
Barriers to implementation
The main barrier to implementation is cost. Some concerns may be also raised about visual intrusion of new signs. However, by reducing the number of stationary road signs it is here suggested that properly designed VMS can reduce negative aesthetic impacts. Long distance car commuters likely to benefit the most, which may raise political issues of disproportionate advantaging of this group in the future.
Text edited at the Institute for Transport Studies, University of Leeds, Leeds LS2 9JT