Truck Platooning

Convoy

We gonna roll this truckin' convoy
'Cross the U-S-A.

How should Travel Demand Models be modified to address Truck Platoons?

Truck platoons are being discussed as a new and innovative way to reduce fuel consumption, reduce air pollution and CO2 emissions and to increase highway capacity.  Given these promises it is natural to try to accommodate trucking platoons in Travel Demand Models that forecast traffic volumes in response to changes in capacity.  However the idea behind truck platoons is not new. C. W. McCall’s classic song “Convoy” was released way back in 1975.  What is new is the technology that might make truck platoons, i.e. convoys, operate safely.

It might be useful to first discuss the concept of platooning.  In NASCAR racing the concept is called drafting.  Drafting is based on proven aerodynamics.  Closely spaced vehicles traveling in a platoon will consume less energy than a vehicle traveling alone.  This is not only true for vehicles, it is also why geese fly in a closely spaced V formation, where the spacing between geese conserves energy. However while the overall energy that is consumed is lower, those benefits are not evenly distributed.  The lead goose, achieves lower savings than all other geese, in the formation. Similarly the lead vehicle in platooning will also achieve lower savings.

source: http://nascarnation.us/page/nascar-sprint-cup-draft-and-aero-explained

According to Wikipedia, on the show Mythbusters drafting behind an 18-wheeler truck was tested and results showed that traveling 100 feet behind the truck increased overall mpg efficiency by 11%. Traveling 10 feet behind the truck produced a 39% gain in efficiency. Additionally, on the same episode, Mythbusters demonstrated that it can be very dangerous for a following vehicle if one of the truck's tires delaminates.  Then chunks of ejected rubber can be large enough to cause serious harm, even death, to a driver following too closely.

In addition, if the vehicle in front stops suddenly, there is also little time to react and stop safely. Truck platooning is proposed as a way to use technology to reduce the safe following gap between trucks. This was discussed during the March 2017 session of FHWA’s’ Talking Freight webinar series.  The recommended safe gap between trucks without platooning is 7 to 8 seconds.  At 60 MPH, the distance associated with this time gap would be approximately 600 to 700 feet.  A normal passenger vehicle such as a car will normally take approximately 320 feet to come to a complete stop after recognizing the need to stop. In comparison, a truck and trailer takes about 525 feet before it comes to a complete stop after recognizing the need to stop. A 700 foot gap should accommodate both the reaction time and a safe stopping distance for trucks.

During that Talking Freight webinar, research was presented for trucks operating in platoons.  It was suggested that for two truck platoons, fuel savings of 10% on rear truck and 4.5% on front truck could be achieved at gaps of 40 feet.  These, gaps which only could be safely achieved with the proposed new technology, which includes vehicle to vehicle (V2V) communication by trucks in the platoon.

As currently envisioned, trucks with the suitable technology would operate as platoons on a completely voluntary basis.  If weather conditions were not favorable, platooning would not be offered.  It would only be offered on controlled access highways operating at speeds in excess of 50 MPH.  If these conditions were not present, then platooning would not be offered and standard gaps and capacity would be expected.

The assignment modules in Travel Demand Models will consider costs and times that vary as the volume to capacity ratio changes during assignment iterations.  However equilibrium after a number of iterations is not guaranteed if capacity on road links changes between iterations, for example in response to such things such as platooning.  If truck platooning provides no advantages for trucks traveling at slow speeds on uncontrolled, or partially controlled, access highways, then there is no reason to ever change the capacity on those road links.  Even on controlled access links, there is no reason to introduce capacities that would vary because of truck platooning.  The volume delay function in travel demand models, typically produces no changes to cost or times at volume to capacity ratios that are less than 0.7.  When speeds are greater than 50 MPH, those volume to capacity ratios would never be exceeded. Thus at speeds where truck platooning might have an impact in travel demand models, is precisely where truck platooning will not be offered.

Also trucks would always want to be something other than the lead truck in order to achieve the higher fuel savings.  In order to be widely adopted, the technology will need to not only to handle shorter gaps but assure an equitable share of positions within the platoons.  However there is no reason to believe that truck platooning would be offered on links where it could actually impact the capacities that are currently used in travel demand models.  Where it will be offered, it would have no impact.  Rather than including capacities that would vary with speed, which would violate a basic premise of the assignment modules, it would thus appear that there is no reason to modify travel demand models to include truck platooning.