Convoy!

It was front page news this week - trials of lorry "platoons" are to be conducted on public roads by the end of next year. Watch out - the convoy is coming!

The headlines generated by this roads story - some of them hitting newspaper front pages, something only likely to happen in August - sometimes verge on the alarmist. The Daily Mail was the most concerned, filling most of its cover sheet with the headline MOTORWAY LORRIES WITHOUT A DRIVER.

The truth behind the story is much less worrying than that, and actually rather more interesting. And just to be very clear, none of the lorries concerned will be without a driver.

The theory

HGVs are enormous vehicles hauling heavy loads over long distances, and in doing so they consume lots of fuel and pump out a measurable slice of the UK's total carbon emissions. New engines are improving year on year, and efforts are made to transfer freight to rail where possible, but it remains the case that our roads will be bristling with big lorries for a long time to come and the electric technology that's about to revolutionise the car industry is unlikely to spread to trucks until it's much more advanced.

That means that finding efficiencies in other ways could be worthwhile. One way to do that is to allow one lorry to travel in the slipstream of another, reducing the energy it takes to drive it forward and reducing its emissions. In the normal run of things, one vehicle close enough to benefit from the slipstream of another is called tailgating and is rightly considered highly dangerous, so the proposal is to equip lorries with autonomous driving technology and allow them to communicate with each other over wifi.

Where one lorry is behind another, the autonomous technology could take over from the human driver and pull it up close to the one in front, mirroring any speed changes in the first vehicle automatically and instantly. The driver in the front lorry would remain in control at all times. So if the front vehicle suddenly brakes, the one behind brakes at the same time.

The DfT's current plan is to allow up to three lorries to travel in a "platoon", each having a human driver at all times, and to begin with it looks like all the lorries participating in the trial will be of the same (or very similar) type so they have the same performance.

The benefits

Initial tests on private roads and test tracks will establish the best way of working - working out, for example, how closely vehicles can safely follow each other in convoy. The trials will then move to public roads. Locations for trials will be chosen once off-road tests have been done.

If it works, the DfT claims that air pollution would be reduced and fuel efficiency would increase, and those seem to be reasonable aims. The lead vehicle in a platoon is estimated to use 8% less fuel than normal and those following behind 14% less.

Reduced fuel consumption would offset the cost of installing the technology, and at present estimates the cost might be recouped in as little as two years - but only if the vehicle in question spends 50% of its travelling time in a platoon enjoying the benefits of the slipstream. It will be a long time before that's a realistic possibility for most hauliers, but as the price of fuel rises and the cost of the technology comes down it may become a more realistic proposition.

The DfT also claims it'll reduce congestion, though the basis for that is less clear. Certainly lorries in platoon will avoid long, drawn-out overtaking maneouvres to get past each other and perhaps by travelling closer together they'll also be occupying less roadspace, though how noticeably congestion might be improved by groups of three lorries travelling together is not altogether clear yet.

The questions

What's already been raised by concerned voices (and headline writers) is how these platoons might operate on busier roads or around junctions. In other words, three lorries in a conga line is a perfectly sensible idea in isolation, but how would they interact with other traffic?

That is a question that will be addressed in the trials, of course, and the Transport Research Laboratory is an organisation of many decades' standing that understands how roads work so these issues will not be overlooked. But in fact some of the answers are already out there thanks to the results of a TRL feasibility study from 2014 that was finally published this week.

The short answer is that, at this stage, platoons will be restricted to quietly-trafficked sections of motorway and may even operate only at night to ensure traffic levels are low enough. Weather conditions would also be taken into account. Around junctions, the researchers actually seem to expect that lorry drivers within the platoon may need to take control of their vehicle to make space for other traffic to filter through and avoid the platoon becoming an obstacle, with the platoon reforming again afterwards. The report is also very clear that - to prevent it becoming a problem - "under specific conditions [the] platoon would be dissolved".

The paper suggests that platoons should be marked in some way so that other drivers can recognise them, and that special care must be taken to allow for other traffic "intruding" into the platoon. The gap between vehicles is - at this stage - anticipated to be in the region of 6m (18ft), and we have all encountered overconfident or impatient drivers on the motorway who would happily swerve into a space that big without a second thought, so one objective of off-road trials will be to make sure the software can cope with unexpected intrusions reliably and safely.

The AA's concern - repeated in all the papers - is that drivers won't be able to see road signs because long lines of lorries will obscure them. The feasibility study actually suggests that signage could be provided on the convoy vehicles themselves to relay information to other vehicles. That would certainly be an interesting sight.

One thing the report is surprisingly casual about is the length of the convoy. Platoons of five vehicles have already been tested elsewhere - comprising a mix of lorries and cars - and TRL suggest that trialling platoons of more than five vehicles should be possible without serious problems. The maximum length of an HGV is 16.5m (54ft) and the DfT have now stated that the maximum length of a convoy will only be three vehicles, so in reality the platoon will present an obstacle little more than 60m (200ft) in length.

One question that doesn't seem to have been addressed at all yet is what will happen when a platoon comes up behind a slower vehicle and needs to move out to pass it. The lead driver couldn't really be expected to be sure that the lane to their right is clear all the way to the back of the platoon, so will a convoy overtake? This and other real-world concerns are where the 2014 feasibility study reaches the limits of what is known and the trials happening this year and next will take over to find a way to make this technology a workable proposition.

There are a lot of questions still to be answered, which is of course why a trial is going to be run, but the level of hubris surrounding it this week probably wasn't necessary (and may well say more about newsrooms' silly season in August than about genuine public concern).

We'll certainly be watching closely - and we'll keep an eye out for the first appearance of Rubber Duck on the UK's roads. Ten-four!