Manual for Streets geometry for a small cul-de-sac

Designing a short residential street to Manual for Streets cul de sac geometry means starting from the speed you want and working back to the width, the radii and the visibility, rather than starting from a standard detail. That is the whole point of the document. A 5.5 metre carriageway with 6 metre radii and long visibility splays invites 30 mph traffic into a street serving eight houses, and then the design has to fight the speed it created.

What follows is the set of dimensions a small scheme actually has to resolve, and where the local highway authority's own standards will override the national guidance.

What Manual for Streets changed

Manual for Streets, published by the Department for Transport and its partner department in 2007, replaced the older residential road guidance and reversed the priorities. Place comes before movement on a residential street. Pedestrians sit at the top of the user hierarchy, then cyclists, then public transport, then general traffic. Design speed is achieved by geometry and by the character of the street, not by a sign.

Manual for Streets 2, issued by the Chartered Institution of Highways and Transportation, extended the approach to busier streets and dealt with the boundary between a street and a road.

Two practical consequences for a small scheme. Visibility requirements are shorter than the older guidance assumed, because they are derived from the speed the street will actually produce. And the geometry can be tighter, which frees up land.

A caution before you rely on any of it. For a street that will be offered for adoption, the local highway authority's own design guide governs, and many authorities have not fully adopted the national approach. Get the authority's current guide, in writing, before you draw. Where the street will stay private, you have more freedom on geometry and less certainty about long-term maintenance.

Carriageway and footway widths

Widths follow from what has to pass what, and how often.

A width allowing two cars to pass comfortably is enough for a street serving a handful of houses where a refuse vehicle comes once a week. Manual for Streets discusses a 4.8 metre width where larger vehicles need to pass a car, and narrower widths where they do not. Below about 4.1 metres two cars cannot pass, which is acceptable on a short length with forward visibility between passing points and unacceptable on a length with bends.

Footways need to be usable by a wheelchair and a pushchair passing, which puts the working minimum at 2 metres. Pinching a footway to 1.2 metres behind a parked car is the most common defect in small layouts, and it is the one an inclusive access consultee objects to.

Three width decisions worth taking deliberately:

• Whether to have footways on both sides, one side, or a single shared surface.
• Where the visitor parking sits, because parking on the carriageway effectively narrows it and parking in a layby does not.
• Whether service strips and verges are wide enough for statutory undertakers' apparatus, because if they are not the apparatus ends up under the carriageway.

Forward visibility and design speed

Visibility splays are calculated from stopping sight distance, and stopping sight distance comes from speed. Manual for Streets sets out the relationship, with around 25 metres appropriate at 20 mph and more at higher speeds.

For a junction, the splay is described by a distance back from the give way line, commonly 2.4 metres and sometimes less where speeds are demonstrably low, and a distance along the major arm equal to the stopping sight distance. For forward visibility around a bend, the same sight distance is measured along the centre of the lane.

The design discipline is to make the geometry produce the speed you assumed. That means tight corner radii, short straights, a bend or a change of alignment within the length, and a surface and enclosure that signal a residential street. A long straight with generous radii produces higher speeds, and a splay drawn for 20 mph then becomes an under-provision.

Check visibility in three dimensions, not two. A splay drawn on a plan over a verge that will carry a 1.2 metre hedge is not a splay, and the maintenance of planting inside splays is something the authority will want secured.

Turning heads for service vehicles

The turning head is the item that most often forces a layout to change late.

Design it around the largest vehicle that has to use the street regularly, which for a small residential cul-de-sac is the refuse vehicle rather than a fire appliance, though both need checking. Ask the authority's waste service for the dimensions and turning characteristics of the vehicle it actually operates, because they differ between authorities and between contracts.

Then confirm three things:

1. That the vehicle can turn within the adopted area using a swept path analysis, drawn and submitted, rather than a standard hammerhead copied from a guide.
2. That reversing is limited to the distance the authority accepts, since many waste services set a maximum reversing length and will refuse to serve a street that exceeds it.
3. That bin presentation points are within the carry distance the building regulations guidance and the authority's own standards allow, both from the dwellings and from the vehicle stopping position. Our note on bin storage and collection points in flatted schemes covers the related arrangement for apartments.

A fire appliance also needs access to within a stated distance of the dwellings, and that is a building regulations requirement with its own dimensions, so coordinate the two rather than assuming one covers the other.

Shared surfaces and when they work

A shared surface, where pedestrians and vehicles use one level surface without a kerb, works on a short street with very low traffic flow, low speeds, a strong sense of enclosure and an obvious entry threshold that tells drivers they have arrived somewhere different.

It works badly where flows are higher, where the street is used as a through route, where parking is unmanaged, or where there is no clear edge for a visually impaired person to follow. National thinking has become more cautious about level surfaces for exactly that reason, and an inclusive access response is now a normal part of the consultation.

If you propose one, evidence it. Show the traffic flow, the length, the entry treatment, a delineator or corduroy surface giving a followable edge, and how parking is prevented from blocking the route. And do not propose one because it saves a kerb line.

Drainage, levels and the detail that gets missed

Levels are where a geometrically correct street becomes unbuildable.

Set the longitudinal gradient so that water runs to gullies rather than to the turning head low point, and check the crossfall against the accessibility requirement for the footway, because a crossfall comfortable for drainage is uncomfortable for a wheelchair. Check that driveway gradients from the carriageway to each threshold work with the finished floor levels, and that a level access threshold is achievable at every plot. And check the tracking against the levels, because a swept path that works in plan can ground a vehicle in section.

Dropped kerbs, tactile paving at crossing points and the transition from footway to carriageway all need drawing rather than noting.

Reconciling national guidance with local standards

The order of precedence in practice is this. The local highway authority's design guide for anything to be adopted. Then the national guidance where the local guide is silent or where you are arguing for a departure. Then the building regulations for access and fire.

Where you want a departure from the local guide, ask for it early, in writing, with the national guidance cited and the reason stated. Highway authorities grant departures more often than people expect when the request arrives before the layout is fixed and comes with a drawing.

For the planning application itself, a small residential scheme will normally need a Transport Statement rather than a full assessment, covering access, visibility, parking, servicing and refuse. Where works to the existing highway are needed to form the access, those are delivered under a separate agreement, as our note on section 278 agreements explains, and parking provision including accessible bays needs to follow the standards set out in our piece on accessible and blue badge parking.

This is general guidance, not legal advice. Check the position with your local planning authority and highway authority, or a qualified transport engineer, before relying on it.

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