Highway Engineering

6m single carriageway road with a high level of access to it and narrow verges

(S2, 6 m, H, 0.5 m verge). Where the exact conditions as defined on the table do

not apply, interpolation between the given figures can be employed.

Alignment constraint

Alignment constraint measures the degree of constraint resulting from the

alignment of the highway. It is assessed for both dual carriageways and single

carriageways:

Dual carriageways:

Ac=6.6 +B/10 (6.1)

Single carriageways:

Ac= 12 - VISI/60 +2B/45 (6.2)

where

B =Bendiness in degrees per kilometre (°/km)

VISI =Harmonic mean visibility

VISI can be estimated from the empirical formula:

Log 10 VISI =2.46 +VW/25 -B/400 (6.3)

where

VW =Average verge width averaged for both sides of the road

B =Bendiness in degrees per kilometre (°/km)

An illustration of the method for calculating bendiness in given in Fig. 6.5.

Having determined values for the two parameters, the design speed is then

estimated using Fig. 1 from TD 9/93, represented here in Fig. 6.6.

New/upgraded rural roads

In these instances, the design speed is derived in an iterative manner, with

an initial alignment to a trial design speed drawn and the alignment constraint

158 Highway Engineering

Road type S2 WS2 D2AP D3AP D2M D3M

Carriageway width 6 m 7.3 m 10 m Dual Dual Dual Dual

7.3 m 11 m 7.3 m* 11 m*

Degree of access H M MLMLM L L L L

and junctions

Standard verge 29 26 23 21 19 17 10 9 6 4 0

width

1.5 m verge 31282523

0.5 m verge 33 30

Table 6.4Layout constraint values (Lc)

* Plus hard shoulder