Vehicle separation failure sometimes allows free ride, sometimes doesn't (CORRECTION)

A failure in vehicle separation may, or may not, give the following vehicle a toll-free ride. In a commentary on the controversy in Massachusetts we wrote that an overcharge due to a failure in vehicle separation will lead to an overcharge of the leading vehicle and a toll-free trip for the follower.

That was an oversimplification, and plain wrong in many instances. Our understanding now is that Massachusetts business rule is that straytranspodner reads DO incur a toll, based on the transponder regostered class. (2009-08-19 13:00)

Whether the toll system uses a treadle, a plain loop or smart loops in the pavement, a curbside vertical light curtain or an overhead laser or video scanner it will occasionally fail to detect the space between two vehicles, which is called a failure to "separate" them.

The vehicle detection and classification (VDAC) system will frame or "see" two vehicles as one and in the simplest result with two close moving cars will overcharge the front vehicle as a 4-axle vehicle and not toll the second car at all.

However it is often more complicated, a couple of toll engineers have pointed out.

Transponder reads are far more accurate than vehicle separation. So there is a good probability that for every separation failure there will be a good transponder read of both vehicles.

Transponders usually contain a vehicle class as well as the account number, so there's an opportunity to correct the VDAC's vehicle separation error.

Based on transponder reads of two separate vehicles each with a transponder class registration of private car both vehicles can be correctly tolled despite the failure of the VDAC to separate them.

The 4-axle count can be overridden.

Or the tolls can be referred for video review, the camera record of the toll pass pulled up on a screen, and reviewed by a back office person.

What happens depends on business rules, which vary from toller to toller, and which are frequently changed. Generalizations are unwise.

"It's a muddy area," comments one toll engineer.

If transponders are mounted with velcro and motorists move their transponders from vehicle to vehicle they may be using a transponder registered as 'car' on a tractor trailer.  This was a major issue in Houston a few years ago. They had no classification system in their highspeed lanes. Many tractor trailer drivers caught on, registered their transponders for a private car and stuck it on the windshield of their big rig and enjoyed the 75% discount that cars get over 18-wheelers.

The transponder-carried class has another problem.

A motorist may hook a double axle boat trailer to their SUV on occasion. Again in such cases the toller depends on a good VDAC axle count and read and separation to assign the right toll.

In the case of failure of vehicle separation and a closely following car with no transponder then most likely the second vehicle will get a free ride.

An engineer in Illinois describes the arrangements on the Illinois Tollway:

"The second vehicle without an axle count may receive a toll at some agencies, or at least they will on the Illinois Tollway.  The Mark IV antenna will still get at a tag read for the second closely following vehicle even if our 4-loop IDRIS configuration does not separate the vehicles.  If the lane controller is unable to assign that tag to a vehicle, based on IDRIS input, the IAG tag read will be flushed up – an RT26 on ETCc’s setup.  (ETC designed the Illinois system)

"The Illinois Tollway does apply a toll to the account based on the default vehicle class assigned to that tag.  If no tag is present of if the tag happens to be invalid, such as not registered to an open account, then due to the miss-detection of the vehicle in the first place there likely is not a good set of violation images.  In this last instance, the second vehicle may get free passage."

We understand the Mass Pike operates witha  similar business rule. (ADDITION 2009-08-19 13:00)

see earlier report:

http://www.tollroadsnews.com/node/4303

The illustrations above come from a Wilbur Smith Associates report.

All-electronic tolling

Massachusettts Turnpike is entirely lane-based, often slow speed electronic tolling, where the challenges of getting good vehicle separation reads are the greatest. Often 'FAST LANE' so-called is 'TOO SLOW LANE' to work right.

All-electronic or open road tolling with vehicles generally spread out will see fewer separation detection difficulties. Of course it has difficulties of its own.

TOLLROADSnews 2009-08-17