#61 Variable speed limit (VSL) effects and challenges
- 3 days ago
- 2 min read
As reported in a previous brief on smart traffic management technology, variable speed limits (VSL) have been shown to significantly reduce crash rates while simultaneously improving traffic flow, particularly in congestion scenarios. VSL adjusts speed limits in real-time based on triggers such incidents, adverse weather and congestion, producing crash reductions of up to 34% according to some studies1. Smart motorways with VSL systems are increasing in number across Australian jurisdictions, which can be expected to benefit road users regardless of vehicle type and technological sophistication. However, at the same time, the number of vehicles equipped with traffic sign recognition (TSR) as a component of advanced driver assistance systems (ADAS) is also increasing. Austroads highlighted these and related issues in a 2020 report2 addressing the challenge of readability of electronic signs for TSR-equipped vehicles.
TSR-equipped vehicles deliver warnings to drivers if they exceed speed limits, and in the case of autonomous or semi-autonomous vehicles may regulate vehicle speed for compliance. It is therefore critical that TSR systems can accurately and reliably read VSL signs in order to provide appropriate responses. The Austroads report subsequently identified problems with VSL sign readability by TSR systems, predominantly associated with ‘LED flicker’:
Image artefacts may be caused by objects with changing or flickering illumination in an image frame and may include missing parts of an object, edge colour artefacts, and object distortion (Silsby, 2015). Flicker can not only be variable across different sign types (manufacturer, or design), but also be out of sequence across different sections of the signs (Austroads, 2018).
Some of the readability issues identified were reportedly addressed in the revision of relevant Australian Standards (AS 4852.1, AS 4852.2 and AS 5156), but it is clear that vehicle manufacturers, road authorities and the traffic management industry need to work on this collaboratively to achieve desired outcomes. For road authorities, it was recognised that changes and upgrades to traffic management assets would essentially need to be incremental and staged through a transition process as determined by resources and priorities. The objectives and costs are highly variable according to jurisdictional requirements:
The potential enhancements that may be deployed are variable and subject to a scalable implementation to increase compatibility with TSR systems. This could range from simple firmware updates in assets to more complex replacements of hardware. The cost of implementing these enhancements would vary from a million to tens of millions of dollars depending on the manufacturer, model and number of devices deployed by each jurisdiction.
As TSR-equipped vehicles continue to increase as a proportion of the total vehicle fleet, research updates on VSL readability and associated issues are critical to maximise the benefits of related network upgrades.
[2] Austroads (2020). Guidance and Readability Criteria for Traffic Sign Recognition Systems Reading Electronic Signs (AP-R627-20). https://austroads.gov.au/publications/traffic-management/ap-r627-20
