(The suffixes A and B indicate the higher and lower categories respectively
within each speed band.) These values are given in tabular form in Table 6.3.
The minimum design speed for a primary distributor is set at 70A km/hr.
Geometric Alignment and Design 157
Speed limit Design speed
mph km/h km/h
30 48 60B
40 64 70A
50 80 85A
60 96 100A
Table 6.3Design speeds
for urban roads
6.2.3 Rural roads
The design speed is determined on the basis of three factors:
The mandatory constraint
The layout constraint (Lc)
The alignment constraint (Ac).
Statutory constraint
The general speed limit for motorways and dual carriageways is set at 70 mph
(112 km/hr), reducing to 60 mph (96 km/hr) for single carriageways. The use of
these mandatory speed limits can restrict design speeds below those freely
achievable and can act as an additional constraint on speed to that dictated by
the layout constraint Lc.
Layout constraint
Layout constraint assesses the degree of constraint resulting from the road
cross-section, verge width and frequency of junctions and accesses. Both car-
riageway width and verge width are measured in metres. Density of access is
expressed in terms of the total number of junctions, laybys and commercial
accesses per kilometre, summed for both sides of the road using the three grad-
ings low, medium and high, defined as:
Low =Between 2 and 5 accesses per kilometre
Medium =Between 6 and 8 accesses per kilometre
High =Between 9 and 12 accesses per kilometre.
The layout constraints for different combinations of the above relevant para-
meters are defined in Table 6.4 for seven different road types.
Having sketched a trial alignment on paper, Table 6.4 is utilised to estimate
Lc,whose value will range from zero for a 3-lane motorway (D3M) to 33 for a
