Highway Engineering

(The appropriate value for desirable minimum radius given by TD 9/93 and illus-

trated in Table 6.9 is almost identical – 1020 m.)

Taking a design speed of 85 km/hr:

R=0.07069(85)^2

=510.7 m

(Again, the appropriate value for desirable minimum radius given by TD 9/93

and illustrated in Table 6.9 is almost identical – 510 m.)

It can be seen that, as with sight distances, the absolute minimum values of

R, consistent with a superelevation of 7%, are one design step below the desir-

able minimum levels. These are termed relaxations. One further design step

below these absolute minima, also termed limiting radius values, is given in Table

6.9. These represent departures and are associated with situations of exceptional

difficulty.

6.5.3 Horizontal curves and sight distances

It is imperative that adequate sight distance be provided when designing the hor-

izontal curves within a highway layout. Restrictions in sight distance occur when

obstructions exist, as shown in Fig. 6.12. These could be boundary walls or, in

the case of a section of highway constructed in cut, an earthen embankment.

170 Highway Engineering

obstruction

SD

Centreline of

highway

Centreline of

inside lane

Ms

R R

Ds

Figure 6.12Required clearance for sight distance on horizontal curves.