Two designs for the same junction can look very different depending on which document was open when they were drawn.
Manual for Streets and the Design Manual for Roads and Bridges cover overlapping ground and start from different assumptions. Applying the wrong one produces a design that is internally consistent and still gets rejected, usually late.
Knowing which applies, and why, saves a round of revisions.
They were written for different problems
The DMRB was developed for the trunk road and motorway network, where the priority is moving traffic safely at speed over long distances. Its geometry reflects that. Generous radii, long visibility, wide carriageways, layouts designed around vehicles.
Manual for Streets came later and addressed a different problem. Residential and lightly trafficked streets are places and not only routes, and applying trunk road geometry to them produces environments that encourage higher speeds and work poorly for people on foot.
So Manual for Streets deliberately allows tighter geometry, shorter visibility distances in appropriate circumstances, and layouts where the design speed is shaped by the street rather than imposed on it.
Which one applies where
Broadly, the DMRB governs the strategic road network and the roads designed to similar standards. Manual for Streets governs residential streets and lightly trafficked roads with a place function.
The difficulty is everything in between, and a good deal of development sits there. A distributor road, a busy urban route with frontage development, a site access onto a road that carries real traffic but sits in a village.
For those, the answer usually comes from the local authority rather than from either document. Many highway authorities publish their own design guidance setting out which they apply and where, and some have local standards departing from both.
That is why asking early beats assuming. A design worked up on the wrong basis is a redesign, not an amendment.
Visibility is where the difference bites
Of all the areas, visibility splays produce the biggest divergence.
The DMRB approach generates longer stopping sight distances, appropriate to higher speed environments. Manual for Streets sets out an approach producing shorter distances in lower speed conditions, on the basis that drivers in those environments behave differently.
The practical consequence is significant. A splay calculated one way may need land the applicant does not control. Calculated the other, it may sit entirely within the frontage.
That single difference decides whether some sites work at all, which is why it belongs at site access appraisal stage rather than at detailed design.
Speed has to be measured, not assumed
Whichever document applies, the design speed comes from what traffic actually does.
A speed survey establishes the recorded speeds, and visibility requirements follow from those rather than from the posted limit. A road signed at 30 where traffic runs at 38 requires the splay for 38.
This catches people in both directions. Some sites need more visibility than the signed limit suggests. Others need less, because traffic is already constrained by the street environment.
Surveying before designing is cheaper than designing before surveying.
Geometry and the vehicles using it
The two approaches produce different junction geometry, and both have to accommodate the same vehicles.
Tighter radii under Manual for Streets are intended to keep speeds down at the junction. They still need to allow a refuse vehicle to turn without mounting the footway, which is where swept path analysis comes in.
A design that satisfies the geometry document and fails the tracking exercise is a common outcome, particularly where the two were done in sequence rather than together.
Where the argument usually happens
A few recurring flashpoints.
An applicant designs to Manual for Streets to make a constrained site work, and the authority applies DMRB standards because of the road’s classification.
An authority applies DMRB visibility to a street that plainly functions as a residential street, producing a design at odds with the character of the area.
Local design guidance departs from both, and neither party notices until technical approval.
A scheme designed under one document is amended under the other during revisions, producing an inconsistent drawing.
Most of these resolve with evidence. Speed data, an accurate description of the road’s function, and a clear statement of which guidance has been applied and why.
Getting it settled early
The useful sequence is straightforward. Establish the road’s classification and function. Ask the authority which guidance they apply to it. Survey the speeds. Then design.
Doing that in the wrong order is what produces the late redesign, and it is avoidable. This is one of the things pre-application engagement is genuinely useful for, since it settles the basis before anyone commits drawing time.
Whichever document applies, the design will go through a road safety audit that examines it on its own terms, and eventually through a Section 278 or Section 38 agreement.
If you have a site where the design basis is unclear, that is worth resolving before design starts. You can reach the team through the London office, the Bristol office, the Cambridge office, the Norwich office or the Welwyn Garden City office.
