Maximum highway gradient

DMRB TD 9/93 Maximum gradient

From Clause 4:

Situation Desirable Absolute
Motorways 3% 4%
Dual Carriageways 4% 8%
Single Carriageways 6% 8%

AASHTO Green Book

Maximum grades of about 5% are considered appropriate for a design speed of 110 km/h [70 mph].

For more detailed guidance, the Green Book provides design curves to calculate the increased accident rate associated with trucks needing to slow below the average running speed of traffic to climb the gradient.