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Safer Roads Unit. Wet Weather VAS at Friction Deficient Sites. Wet Weather VAS. Friction supply is rarely lower than demand in normal, dry conditions and as such advisory speeds set using friction supply would be expected to have poor compliance.

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Safer Roads Unit

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Safer roads unit

Safer Roads Unit

Wet Weather VAS at Friction Deficient Sites

Wet weather vas

Wet Weather VAS

  • Friction supply is rarely lower than demand in normal, dry conditions and as such advisory speeds set using friction supply would be expected to have poor compliance.

  • Wet weather is considered to be the most common and most readily treatable cause of a friction deficiency. For this reason it was felt that some targeted form of speed management should take place at sites that have a wet friction deficiency and/or associated crash history.

  • The ultimate remedial measure for friction deficient sites is to increase the friction supply of the site through pavement works. However, such works might not be able to be implemented immediately due to various factors such as funding restraints and the lead time required for the design and implementation of the scheme.

  • In order to reduce the risk of more friction related crashes from happening in the interim period, temporary use of Vehicle Activated Sign (VAS) warning sign would be a relatively inexpensive countermeasure.

Vehicle activated signs vas

Vehicle Activated Signs (VAS)

VAS units are discrete signs, which remain blank until activated by an approaching vehicle exceeding a preset speed limit, or `trigger speed‘.

Vas display arrangement

VAS Display Arrangement

Solar panel

Rain Detector


LED display


Familiarity: Drivers will respond better to signs that are familiar to them. VAS can use established warning and regulatory diagrams from the MUTCD which are automatically recognisable to the motorist.

The identification and treatment process

The Identification and Treatment Process

Vericom g force meter

Vericom G force Meter


  • a US company that produces multi-axis accelerometers (G-force meters) capable of recording acceleration or G-forces at a frequency of 100 readings per second (100 Hz), and is the current preferred supplier of these products to TMR

  • a portable device for ‘in vehicle’ use that is highly suited for crash investigation purposes at site specific locations

  • the colloquial term for these devices within TMR

Vericom friction testing

Vericom Friction Testing

Friction supply

  • Is the amount of friction that can be developed between the road surface and a vehicle’s tyres

  • It can also be described as the available friction

  • It is obtained by performing a slide-to-stop (skid) test at the crash site, where G - force is measured as a deceleration of the test vehicle on the nominated pavement surface

  • An Interim Test Method is available for a Slide-to-stop (skid) test for measuring surface friction supply using a Vericom device

  • TMR commissioned ARRB Group Ltd to develop this test methodology to ensure acceptable accuracy and repeatability of test measurements

  • This Interim Test Method is a working document and is expected to be finalised over the coming months

Vericom friction testing1

Vericom Friction Testing

Friction Demand

  • Is the vector sum of the lateral and longitudinal g-forces acting on a vehicle when negotiating a certain feature or obstacle

  • It could also be described as the friction required from the road surface to perform a particular manoeuvre at a given speed without loss of vehicular control

  • Is heavily dependent on the manner in which the vehicle is being driven (for example, travel speed and driver inputs such as steering, braking and acceleration)

  • Therefore, the Normalised Friction Demand is obtained by driving through the crash site simulating normal driving

  • The vector sum G force is calculated using the lateral G force (Gy) from the drive-thru and a default longitudinal G force (Gx)

  • The default Gx of 0.2 provides for moderate braking or acceleration friction demand

Vericom friction testing2

Vericom Friction Testing

  • Vericom testing is conducted at the crash site to evaluate the factor of safety between the friction supply and the friction demand

  • If the Factor of Safety is achieved, the surface friction is considered adequate for the site

  • If the Factor of Safety is not achieved, there is only a small safety margin between the friction supply and demand values

  • If the normalised friction demand exceeds the friction supply for a given travel speed, then loss of control is expected to occur

  • In the latter two scenarios, intervention is required to ensure safety

Vas activation speed

VAS Activation Speed

Wet and dry

Wet and Dry

  • Installation of warning sign with SLOW DOWN” VAS for sites with wet weather friction deficiency .During wet weather, the VAS is to be activated at a speed that meets the minimum Factor of Safety requirements based on friction supply and demand testing. The dry activation speed is then set at 10kmh above the wet setting.

  • The wet weather sensor is used to change the ‘speed trigger threshold’ for approx 30mins after rain is detected.

Reference documents

Reference Documents

  • TMR Skid Resistance Management Plan.

  • TMR Guide lines for Site Specific Friction Supply & Demand Safety Assessment.

  • TMR paper - 2010 Australasian Road Safety Research, Policing and Education Conference 12 31 August - 3 September 2010, Canberra, Australian Capital Territory.

  • Austroads Guide to Management of Road Surface Skid Resistance, AP-G83/05, 2008.

Safer roads unit



Safer Roads……. “Sooner rather than Later”

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