In practice, there’s no legal height limit for trailers in the UK. However, bridges spanning roads that measure over 16.5ft (5.03m) don’t have the height marked on them.
To carry the maximum permitted gross combination weight (GCW) of 44 tonnes in the UK, both tractors and semi-trailers must have three or more axles. Vehicles heavier than this aren’t allowed on UK roads, except for indivisible loads classed as abnormal or oversize. These vehicles must display a Special Types General Order (STGO ) plate on the front of the prime mover and may be required to travel an authorised route under escort. In the UK, this is mainly carried out by private companies. However, extremely large or heavy loads that require road closures must be escorted by the police.
Most UK trailers are 45 feet (13.5m) long. Depending on the position of the fifth wheel and kingpin, a coupled tractor unit and trailer will have a combined length of 50 to 55 feet (15.25 to 16.75 metres). The Construction and Use Regulations allow a maximum rigid length of 60 feet (18.2m). When combined with a shallow kingpin and fifth wheel set close to the rear of the tractor unit, this can give an overall length of around 75 feet (22.75m). Combinations of this length are very unusual, although concrete or steel beams may be carried using this combination.
Since 2009, the consequences of increasing semi-trailer length in the UK have been under consideration. This would make UK trailer lengths fit with with EU directives.
The maximum overall length in the EU and EEA member states is 18.75m, with a maximum weight of 40 tonnes – or 44 tonnes if carrying an ISO container. However, rules limit semi-trailers to 16.5m, or 18.75m if a truck is carrying a standardised 7.82m body with one additional 7.82m body as a trailer. These are referred to as drawbars and hadn’t been widely used in the UK until recently.
Several variations that could take combination lengths up to 25.25m are being considered in Europe. The drive behind these large combinations is environmental: with a 50% increase in cargo weight, fuel efficiency increases by an average of 20% and brings with it a decrease in CO2 emissions. If trailers were this size, the number of trucks on the road would reduce by a third. The plans for making trucks of this size legal in the EEA are referred to as EuroCombi.