Moving Packaged Treatment Plant and Plant Rooms by Road: UK Dimensional Limits, Abnormal Loads and Module Sizing

The normal limits: Construction and Use and Authorised Weight

Any vehicle and load that fits within the Road Vehicles (Construction and Use) Regulations 1986 and the Road Vehicles (Authorised Weight) Regulations 1998 travels without notice to anyone. Regulation 8 caps the width of a motor vehicle or trailer at 2.55 m (2.6 m for refrigerated bodies); regulation 7 caps a rigid vehicle at 12 m, an articulated vehicle at 16.5 m with a 13.6 m semi-trailer, and a drawbar combination at 18.75 m. No general height limit exists except 4.57 m for buses, but regulation 10 requires a cab notice of the travelling height whenever it exceeds 3 m, and regulation 10(4) only exempts journeys on which every structure clears the vehicle by at least a metre. Weight under the 1998 Regulations tops out at 44,000 kg for a six-axle articulated combination with road-friendly suspension and a drive axle of no more than 10,500 kg, 40,000 kg on five axles and 38,000 kg on four; a kingpin-to-rear-axle check of 5,500 kg per metre can bite on short trailers.

Loads wider than the vehicle are allowed under regulation 82, with conditions that escalate with width and length:

Notification periods are two clear working days, excluding Sundays and bank holidays, and the notice must state the time, date and route and the dimensions of each vehicle and projection. Police forces may accept less at their discretion but seldom do for first-time movements.

Special types: STGO categories and weight

Loads that cannot be carried within the Authorised Weight Regulations move under the Road Vehicles (Authorisation of Special Types) (General) Order 2003 as abnormal indivisible loads. Three categories apply, each requiring a plate on the tractor and its own speed limits:

• Category 1: up to 46,000 kg on five axles or 50,000 kg on six, with axle weights within the normal regulations; 60/50/40 mph on motorway, dual carriageway and other roads.
• Category 2: up to 80,000 kg on at least six axles, 12,500 kg per axle; 40/35/30 mph.
• Category 3: up to 150,000 kg, 16,500 kg per axle; 40/35/30 mph.

Article 17 requires notice to every road and bridge authority on the route, with a signed indemnity for damage, whenever the gross weight exceeds 44,000 kg or axle weights exceed the Authorised Weight Schedule 3 limits; the period is two working days for Categories 1 and 2 and five for Category 3. Above 80,000 kg the police must also be notified. Most packaged plant movements that leave the normal regime do so on weight rather than width: a 14 m plant room with internal tanks and a concrete floor slab can reach 40 t, which on a 17 t tractor and step-frame trailer gives a 57 t train and a Category 2 movement even though the width may be within 3 m.

ESDAL and the notification workflow

National Highways’ Electronic Service Delivery for Abnormal Loads (ESDAL) is the online system through which hauliers plan a route and notify police, highway authorities and structure owners in one submission. A load is notifiable through ESDAL when it exceeds 44,000 kg, 2.9 m width or 18.65 m rigid length. ESDAL holds the structures database, flags bridges whose assessed capacity or headroom is below the proposed load, and returns objections or conditions from each authority. Lead time in practice is a week for a routine Category 1 or 2 movement on a known route and several weeks where a bridge owner (Network Rail, a canal trust or a local authority with a weak structure) has to assess the load individually. Special orders for loads above the STGO ceilings take months and are costed in tens of thousands of pounds once escorts and traffic management are included.

Escorting is a matter of police policy rather than statute. Since 2010 the police have withdrawn from most routine escorts, and self-escort by trained private escort vehicles is normal for loads above 3.5 m wide or 25.9 m long. Movements above 5 m wide or 100 t often still require police presence, dictated in the ESDAL response, with the cost recoverable from the haulier.

Height: no law, but a bridge

Height is governed by structures, not by the regulations. Design Manual for Roads and Bridges CD 127 sets the headroom at new overbridges on motorways and trunk roads at 5.30 m plus a sag-curve allowance of up to 80 mm, and the maintained headroom, the minimum to be preserved at all times, at 5.03 m; footbridges, sign gantries and other structures vulnerable to impact get 5.70 m new and 5.41 m maintained, and structures on designated high load routes 6.45 m. Those figures apply to the strategic network. Local roads carry thousands of older arch and rail bridges with signed headroom of 3.5 to 4.5 m, and railway bridges below 4.95 m (16 ft 3 in) must be signed, which is why 4.95 m has become the unofficial threshold between a routine movement and one needing a detailed height survey. Regulation 10 requires the cab notice in feet and inches, and the driver must plan to avoid any structure that does not clear the travelling height by a metre unless a documented route is carried.

Travelling height is the module height plus the trailer deck: about 1.35 m for a flat trailer, 0.95 to 1.0 m for a step-frame and 0.5 to 0.6 m for a low loader. A 3.0 m high module on a step-frame travels at 4.0 m and goes almost anywhere; a 3.5 m module on the same trailer travels at 4.5 m and starts to meet signed bridges on B roads; a 4.0 m module needs a low loader to stay under 4.6 m and will still need a route survey. Overhead power lines are the other constraint: distribution network operators ask to be told of loads over 5.2 m under 11 kV and 33 kV lines, and many rural site accesses have lower crossings than the public road.

What the thresholds mean for module sizing

Working back from the law gives a hierarchy of preferred module envelopes:

Worked example. A proposed plant room is 14.0 × 3.6 × 3.4 m and 38 t with its filters, pipework and a cast floor. Width 3.6 m exceeds 3.5 m, so police notice and an attendant are needed; the train weight of about 55 t makes it STGO Category 2, with ESDAL notification and indemnities to every highway and bridge authority on the route and a 40 mph motorway limit; the travelling height of 4.4 m on a step-frame requires a route survey and a cab notice; the 14 m load overhangs a 13.6 m trailer by 0.4 m, within the rear projection rules. Splitting the same plant into two modules of 7.0 × 3.0 × 3.0 m at 19 t each brings both inside the normal regulations except for a 3.0 m width, which needs police notice only; each travels at 4.0 m on a step-frame and at 44 t gross on a standard tractor unit. Reconnecting pipework, cabling and cladding at the site joint costs perhaps two days of fitting, against a haulage saving that typically runs to several thousand pounds per movement and, more importantly, removes the programme risk of a refused route.

Design rules that follow: keep width at 2.9 or 3.5 m rather than 3.0 or 3.6 m, because the thresholds, not the round numbers, set the cost; put the split line through a corridor rather than through a tank; ship tanks empty and ballast on site; keep the centre of gravity low and recorded on the lifting plan, since a top-heavy module forces a low loader for stability before it does for height; and check the site access, not only the public highway, for the last 200 m of bends, gradients and overhead services. For the plant room designs themselves and how envelopes are chosen for different process trains, see the packaged plant rooms page.

Pitfalls

• Measuring the module, not the load. Width is measured over the widest point including cladding flashings, lifting lugs, door handles and eaves gutters; a 2.9 m module with 60 mm gutters is a 3.02 m load.
• Ignoring the lateral projection rule. A 2.55 m trailer carrying a 3.0 m module has 225 mm projections each side, under the 305 mm limit, but the overall width still exceeds 2.9 m and triggers notice.
• Assuming Scotland and Wales match. Trunk road authorities differ (Transport Scotland, Welsh Government), ESDAL covers England, Scotland and Wales but local police procedures vary, and Northern Ireland has its own abnormal load regime.
• Forgetting the crane. A 38 t module lifted at 12 m radius needs a 200 to 250 t mobile crane, itself an STGO Category 2 or 3 movement with its own notifications and ground bearing requirements.
• Chemical inventories. Modules shipped with dosing chemicals on board bring the carriage of dangerous goods regulations into play; ship them empty.