Southern M90 Forth Road Bridge approach speed limit: FOI release

FOI reference: FOI/17/01824
Date received: 10 August 2017
Date responded: 21 September 2017

Information requested

[* FRB - Forth Road Bridge]

You asked for information regarding the following:

1. Which legislation allows TS to vary the speed limit on the M90 southern FRB approach before the 40mph limit at roadworks.

2. Who determines the variable speed limit.

3. What conditions are in place to verify that the speed limit is reasonable.

Response

1. The variation of the national speed limit in the location you describe is enacted through the M9/A90/M90 Trunk Road (Kirkliston to Halbeath) (Variable Speed Limits and Actively Managed Hard Shoulder) Regulations 2012.

2. A monitoring and control system covering all motorway variable speed limit indicators across Scotland is in place and in continuous operation.

This system (Traffic Scotland) provides a framework of monitoring and control underpinned by national specifications and standards to ensure imposed speed limits are at all times reasonable and fully aligned with agreed operating procedures.

The system provides a real time display of the speed limit set on all the deployed equipment at the Traffic Scotland National Network control centre and maintains a permanent log recording when and for what reason every speed limit setting is applied.

Variable Speed limits are activated for a number of different reasons:

• As part of the response to an Accident, Road Closure, Breakdown, Roadworks or other hazard impacting the road where reducing traffic speed helps to make the road safer.

• As part of the management of traffic congestion reducing the speed limit when traffic is starting to build up to smooth traffic flow and avoid the formation of queues (Variable Mandatory Speed Limits) and once queues form slowing traffic on the approach to the queue to reduce sudden braking and accidents (Queue Management).

Most of the speed limits set are activated as part of a system recommended Incident response following agreed response rules maintained in the system which are tested and approved in advance by simulating and checking several thousand event scenarios, covering different types of Incidents, Accidents, Breakdowns, Queues etc. at different points in the road network.

3. In all cases requests to apply speed limits are verified against the Signal Sequencing Rules a UK wide standard (MCE 2103 NMCS2 Signal Setting Rule Specification) which is used to verify all signal setting requests and generate additional speed limit settings both on adjacent lanes and in advance of the gantry being set in line with the rules to ensure that at all times the speed limits set on the whole road network are consistent, follow the rules and are safe.

Responses planned and checked in advance combined with continuous verification under the Signal Sequencing Rules, continuous monitoring by the Traffic Scotland Operator team and full logging ensures effective suitability, verification and audit of applied speed limits.

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