The Highways Agency takes a number of measures to ensure sign accuracy.
The majority of signs are automatic and set by sensors or other roadside infrastructure. As a result any message displayed is real-time and takes account of changing road conditions.
Beyond the automatic signs the remaining messages are set manually by operators in either a regional or the national control centre.
To ensure signs are set in a timely manner, performance indicators have been put in place to monitor how quickly a message is placed on a sign. The indicator requires operators in the regional centres to set or remove legends within two minutes of an incident being confirmed.
Further procedures and alarms mean that checks take place while an incident is running. Procedures in the national centre require operators to review incident “Events” every 45 minutes and alarms are tripped for every three-hour period that a variable message sign is not updated.
Steps are also taken to review sign accuracy. Each month, both the regional and national centres review incidents to assess the overall accuracy of their sign settings, and to identify any opportunities to make improvements.