Due to aerodynamic drag effects one of the most often talked about methods of increasing the efficiency of road transport is for vehicles, whether trucks or passenger cars, to platoon together. Usually it’s proposed that multiple vehicles closely follow behind a lead vehicle that bears the brunt of the aerodynamic drag.

The optimum separation distance, balancing the tradeoff of drag reduction and safety, depends on vehicle height and speed, and could be as little as 2m for shorter vehicles at 60 km/h or up to 9m for full height trucks at 120 km/h. 

This is the primary feature of platooning, vehicles are so close together that they cannot safely operate independently but as a combined ‘train-like’ vehicle. With only one driver determining the platoon speed and direction. Thus necessitating highly coordination actions within the platoon.

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For the example of long haul trucking, the main motive for their introduction is cost savings via overall efficiency improvements which depend on many factors including the size and weight of vehicles, the presence of headwinds, etc. 

For the case of semi-trucks hauling standard 53 foot trailers, in the most optimistic scenario of strong headwinds and a dozen semi trucks platooned, each hauling their maximum gross weight of 82000 lbs (in the U.S.) on a flat slope, mileage could double with the same amount of fuel. 

Clearly halving fuel consumption is an enormous potential benefit as fuel costs can make up the majority of shipping costs nowadays in long range truck hauling.

However, this system has not been introduced yet into any real world truck route for a number of reasons. Some of which are technical, though those have been recently addressed in the last decade, but some are primarily issues of coordination. 

Some of the coordination issues arise from quite deep problems of human behaviour and environmental constraints.

One of the most immediate problems is that trucking companies typically don’t send out a dozen trucks at once, but one or two every so often. So realizing greater efficiencies would likely necessitate some kind of agreement between several trucking companies to send trucks out near simultaneously to platoon.

Competitive dynamics would likely make them hesitant to cooperate. The parallels are obvious enough with any other type of competitive activity that the possible coordination issues are well covered elsewhere.

(e.g. How can truck companies coordinate together to reach an agreement and enforce said agreement? Especially since such agreements necessarily must involve sharing of data such as truck routes, scheduled time, departure and arrival time, etc.)

Beyond problems of competition however, there also are the following:

1. Who gets to be the lead vehicle? How will other companies compensate the lead vehicle owner for their extra costs?
2. If the lead vehicle gets into an accident, or have parts fall off, the trailing vehicles will not have space to safely stop, so all platooned vehicles would likely crash. How would fault and liability be proportioned out? If the worst happens, would the lead vehicle operator be responsible for wrecking a dozen trucks? 
3. In the case of an outside vehicle crashing into one of the platooned trucks does the outside vehicle driver bear the cost of damaging potentially tens of millions of dollars of trucks and cargo? Would car insurance minimums have to be raised by an order of magnitude or more?
4.  In the case of an outside vehicle crashing into the last of the platooned trucks, do all trucks stop or do they continue on except for the last one?
5. A dozen trucks is a very long distance to pass for other vehicles. Do other road users get compensation for the increased inconvenience and hazards of driving? 
6. and so on

These have interesting tie-ins and parallels with existing problems in other areas, such as road pricing, carbon taxation, pollution externalities, insurance systems, policing,  distributed systems, etc.

Importantly, these issues cannot be resolved separately but must all be resolved together before the introduction of viable platoons into our road network.

Whether or not platooning can be successfully introduced is therefore dependent on a huge number of factors and agents working together in harmony.

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Since this is reasonably low hanging fruit, affecting only a limited area, sphere of activity, etc., compared to some other coordination issues, the capacity to resolve vehicle platooning may serve as a litmus test for more ambitious endeavours involving a wider array of parties and/or a broader area of influence.