Can our urban roads cope with longer semi-trailers?



Logistics at its worst: this 13.5m trailer should never have entered our city centre.
Image as described adjacent

In January, a trial started of articulated vehicles with longer semi-trailers, the government having given permission for some 1,800 longer vehicles. 900 of these are allowed to be 2.05 metres longer than the present maximum trailer length of 13.6m.

Although these longer lorries can be no heavier than existing vehicles, and must pass the turning circle regulations as specified in both EU and UK Construction and Use 1986 regulations, these vehicles will pose particular problems in urban areas. There are no proposals to restrict such access.

All normal vehicles, and such trial combinations, are required to pass both a ‘concentric circles’ and a ‘drive-through’ test to supposedly limit off-tracking issues, and although semi-trailers can be constructed to pass the regulations, it is the regulations that fail, especially for long semi-trailers.

It is quite unreasonable to expect other road users to be able to predict the path and off-tracking of such vehicles

For the concentric circles test, a steady state turn with the outside corner of the tractor unit just inside a circle of radius of 12.5 metres is required to have no part of the tractor-trailer combination inside an inscribed circle of 5.3 metres. Simple geometry shows that for a single-axle trailer, the axle needs to be about 8 metres from the fifth wheel (king pin) position for a typical arrangement with a three-axle tractor unit. This would give an overhang of some 6 metres behind the rear axle (see diagram on next page). Such a vehicle would not have much practical use unless all it carried was potato crisps, as the single trailer axle would soon be overloaded, but it is easy to imagine the out-swing should such an articulated vehicle make an especially sharp, but totally practical turn.

For the drive-through test, the combination is required to move from a straight line to follow a tangential arc of radius 12.5 metres. At no time must the out-swing of the rear corner of the trailer be more than 0.8 metres. This is hardly a demanding test, even with our odd single-axle trailer. A typical tractor unit could turn on a radius of 8 metres or less, and the tangential arc rule means that even after a 90 degree turn significant amounts of steering lock remain unused.

To really test the out-swing, and to be more representative of tight urban junctions, full lock needs to be applied over a mere metre or so, with the tractor unit continuing on full lock until it has turned through more than 90 degrees. Then we will get an out-swing well in excess of 2.5 metres. This is just the sort of turn needed to enter a side road with a lane width of 3.75 metres.

So how does this change with a typical longer semi-trailer, carrying normal loads on multiple axles?

Very little. Clearly we need additional axles to carry the weight, but they must not significantly change the position of our ‘effective’ single-axle or the trailer will fail the concentric circle test. The method normally used on existing long trailers tends to have self-steering axles to the rear of a fixed axle. These act a bit like castors, taking a share of the weight but not changing tracking from that of a single-axle trailer. Self-steering axles can be used on the new longer trailers, but another option is to use ‘command’ steer, where the axles are steered according to the angle of articulation. This at least means that, unlike with self-steering, tracking is the same in reverse as in forward, but the out-swing on sharp turns will be equally bad.

Is this not like carrying a long plank on your shoulder? No, it is even worse. At least with a plank you can look over your shoulder and see where the back end is. With these sharp turns the out-swing will always be on the blind side where no cab mirrors, however good, can enable the driver to see.

The TRL-commissioned report on off-tracking issues failed to consider in detail any turns other than those defined in legislation. It failed to use a widely used computer program to model sharp turns, and it failed to look at off-tracking issues encountered by existing 13.5 metre trailers at tight, light-controlled junctions where they commonly use adjacent traffic lanes or cross onto the wrong side of the road.

Currently, legal draw-bar trailers with a rigid lorry can carry almost the same volume of goods as the new longer semi-trailer without serious off-tracking issues. However the distribution industry considers these less flexible, hence the request to government for longer semi-trailers. Is this not a case of the cart leading the horse?

How does this affect other road users? Often, people such as those on bicycles, or even in cars, are blamed in incidents involving long trailers. It is quite unreasonable to expect such road users to be able to predict the complex path and very significant off-tracking of such vehicles. But equally it is quite reasonable for the qualified driver of such a longer trailer to understand how it will off-track. The commissioning of even longer semi-trailers can only make this position worse.

Concentric circles test for articulated vehicle with 15.5 metre semi-trailer
Image as described adjacent

Government ministers have even said in parliament: ‘Because the turning wheels of longer semi-trailers are at the back, their turning circles are much tighter than those of existing lorries’. I fail to understand how that fits with existing regulations. For tight turns where out-swing of existing ones is clearly a problem, unless the longer trailer bends in the middle, or the laws of physics are changed, the extra two metres at the rear must create greater problems.

Of course, such extra-long trial semi-trailers could be restricted to government-funded Highways Agency roads where such tight turns are rarely found. Special permission would then be required when using locally-funded roads. With improved Sat-Nav systems this would be easy and cost-effective to introduce. At least then we might be able to control the damage to footways and street furniture that is an unrecoverable cost for local authorities.

Jim Chisholm

In the late 1970s Jim developed TRACK, the first practical computer model that could predict the path of complex vehicles when manoeuvring. He now campaigns on transport issues and is the volunteer liaison officer with Cambridge Cycling Campaign.