Vehicle swept path analysis for UK housing layouts

A swept path analysis draws the ground a vehicle actually covers as it turns — not the line its wheels follow, but the whole footprint of the body, overhangs included. On a housing layout it decides whether the refuse lorry gets down the street and back out again, and it is a common reason layouts come back marked up.

What a swept path analysis actually is

The Design Manual for Roads and Bridges defines it tightly. CD 123 (Geometric design of at-grade priority and signal-controlled junctions, National Highways) calls a swept path the movement and path of different parts of a vehicle undertaking a turning manoeuvre — the envelope swept out by the sides of the body, or any other part of the structure.

Three things happen at once. The rear axle cuts inside the front axle, by more on a tighter radius and a longer wheelbase. The front overhang swings out past the steered wheels on the outside of the bend. The rear overhang tails out behind, clipping the build-out.

The cut-in is the one people underestimate, and geometry alone fixes it. On a steady turn with front-axle radius R and wheelbase L, the rear axle runs on the square root of R squared minus L squared — an off-tracking close to L squared over 2R. A refuse vehicle with a 5.9 m wheelbase on a 15 m radius bend off-tracks about 1.2 m, the inside rear wheel that far inboard of where the driver put the cab. Add a trailer and the second stage cuts inside the first again.

A vehicle entering a bend, though, is not on a steady circle, and the transient is where layouts fail. The path a dragged axle takes is a tractrix, the seventeenth-century curve of a weight pulled along on a taut string: the trailing point always moving straight towards the leading one, a fixed distance behind. Swap "string" for "wheelbase" and that is a rear axle following a front one. A hand template assumes the steady state, so it is right on a constant-radius bend and wrong wherever the vehicle is still turning in.

Which vehicles UK authorities test against

There is no single national design vehicle for a housing layout. Settle the dimensions before you draw: they decide the answer.

The refuse vehicle. Manual for Streets (DfT, 2007) paragraph 6.8.7 reports BS 5906:2005 as noting the largest waste vehicles then in use were around 11.6 m long. But the vehicle you must satisfy is the one your collection authority runs, and most publish it: Bath and North East Somerset's waste planning checklist asks for tracking on a vehicle 10.65 m long, 2.9 m mirror to mirror, with a 5.25 m wheelbase, 17.7 m of turning space between kerbs and 28 tonnes laden. Others specify heavier four-axle vehicles. MfS 6.8.6 helps if the demand looks excessive: it is neither necessary nor desirable, it says, to design new streets for larger waste vehicles than can already be used nearby.

The fire appliance. Approved Document B Volume 1 (Dwellings) gives you the route, not the appliance. Table 13.1 sets, for a pump, 3.7 m minimum road width between kerbs, 3.1 m at gateways, a 16.8 m turning circle between kerbs (19.2 m between walls), 3.7 m clearance and 12.5 tonnes capacity. Its own first note settles any argument about universality: fire appliances are not standardised, and the building control body may, with the local fire and rescue service, use other dimensions. Paragraph 13.1 asks for access within 45 m of all points inside a dwellinghouse; 13.4 wants turning facilities on any dead-end route longer than 20 m. These are England's figures; the other UK nations differ.

The car. MfS 8.3.58 recommends off-street bays as a rectangle at least 4.8 m by 2.4 m for the vehicle — the origin of the 4.8 m design car, and the figure behind car park geometry.

Goods vehicles. Legal maxima come from the Road Vehicles (Construction and Use) Regulations 1986, summarised in DfT's guidance on maximum vehicle lengths in Great Britain: 12 m rigid, 16.5 m articulated. The Longer Semi-Trailer, made permanent in 2023 by SI 2023/524, allows a 15.65 m loading area within an 18.55 m combination. CD 123's design vehicle is that 16.5 m artic — which matters where your access meets a trunk road, not inside the estate.

When a swept path analysis is required

Tracking is not itself a statutory requirement. It arrives through four doors.

• Planning validation. Most authorities put vehicle tracking on their local validation list for anything beyond a few homes. The NPPF requires development generating significant amounts of movement to be supported by a transport statement or assessment — the paragraph number moves between editions — and tracking is normally an appendix.
• Highway adoption. Where streets will be adopted under section 38 of the Highways Act 1980, technical approval wants tracking against the vehicles in the authority's own design guide. Section 278 works too.
• Building control. AD B's access provisions are tested at building regulations stage, not planning: a consented scheme can still fail on fire appliance access.
• The waste authority. Often a separate consultee with its own checklist and its own vehicle.

MfS 7.10.2 is explicit about one case: where a three-point turn must be provided, typically in a cul-de-sac, a tracking assessment should show which vehicles make the manoeuvre and how.

Where housing layouts usually fail

• Bends that look generous. Off-tracking grows with the square of the wheelbase. A radius a car takes without thinking puts a refuse vehicle's inside rear wheel over the kerb.
• Turning heads. The classic. AD B wants a turning facility beyond a 20 m dead end; MfS 6.8.8 reports BS 5906:2005's recommended maximum reversing distance of 12 m, longer only where the reversing route is straight and unobstructed. MfS 6.8.10 adds that a waste vehicle may not need to enter a cul-de-sac shorter than around 55 m at all.
• Junction kerb radii. Tighten the radius for pedestrians and the lorry mounts the footway; open it out and you have built a slip road into a residential street. MfS 7.2.7 offers the way through: the kerb line need not follow the tracking line, provided sightlines, parking and pedestrian movement are weighed together.
• Parked cars and other pinch points. Tracking a conveniently empty carriageway is the error that survives to site: run it again with the bays occupied, especially on bends. Gate piers eat into AD B's 3.1 m gateway minimum, and street trees, columns and bin presentation points narrow the width where the overhang swings.

How it is done, from templates to the layout tool

For decades it was transparent plastic templates over a paper drawing: fast, cheap, steady-state only. Since the 1990s it has been CAD add-ins — AutoTURN, AutoTrack and Autodesk Vehicle Tracking are the names on most submitted drawings — which simulate the drive line rather than stamp a shape. They are accurate, expensive, and used at the end, by which point moving a turning head means moving plots.

Airship Site Designer runs the same test inside the layout. Pick a design vehicle — refuse (11.2 m), fire appliance (8.3 m), car (4.8 m), rigid HGV (12.0 m), articulated HGV (16.5 m) or the 18.55 m Longer Semi-Trailer — and click points along a route; the drive line follows the road centreline between clicks. The rear axle chases the front at one wheelbase in a pure-pursuit tractrix, and an articulated trailer's bogie is a second tractrix stage chasing the kingpin, so trailer cut-in is simulated, not sketched. Three drive lines are offered — left lane with swing-out, hold left lane, and carriageway centre for a full-width worst case — and the result reports opposing-lane use in metres, marks where the envelope leaves the carriageway, and flags curvature beyond the steering lock. The dimensions and presets behind it are our own defaults, not quotations from anyone's standard, so check both against what your authority specifies; the estate layout workflow and the documentation cover the rest.

Wherever you run it, run it early and keep it subordinate. MfS 7.2.6 says tracking determines the space vehicles need, but buildings and spaces should generally be considered first, with the carriageway fitted into what remains — the case for designing estate roads to Manual for Streets from the first sketch, not the last.