d 2 =Distance between the overtaking and opposing vehicles at the point in time
at which the overtaking vehicle returns to its designated lane (Safety Time)
d 3 =Distance travelled by the opposing vehicle within the above mentioned ‘per-
ception-reaction’ and ‘overtaking’ times (Closing Time).In order to establish the values for full overtaking sight distance, it is assumed
that the driver making the overtaking manoeuvre commences it at two design
speed steps below the designated design speed of the section of highway in ques-
tion. The overtaking vehicle then accelerates to the designated design speed.
During this time frame, the approaching vehicle is assumed to travel towards
the overtaking vehicle at the designated design speed. d 2 is assumed to be 20%
of d 3.
These assumptions yield the following equation:
FOSD =2.05tV (6.6)
where
V=design speed (m/s)
t =time taken to complete the entire overtaking manoeuvre (s)
The value oftis generally taken as 10 seconds, as it has been established that it
is less than this figure in 85% of observed cases.
If we are required to establish the FOSD for the 85th percentile driver on
a section of highway with a design speed of 85 km/hr (23.6 m/s), we can use
Equation 6.6 as follows:
FOSD^85 =2.05 ¥ 10 ¥23.6
=483.8 m
This figure is a very small percentage less than the value given in TD 9/93 and
illustrated in Table 6.8 (490 m).
If we go back to the three basic components of FOSD, d 1 ,d 2 , and d 3 ,we can
derive a very similar value:
d 1 =10 seconds travelling at an average speed of 70 km/hr (19.4 m/s)
= 10 ¥19.4 m
=194 m
d 3 =Opposing vehicle travels 10 ¥23.6 m
=236 m
d 2 =d 3 /5
=47.2 m
FOSD = 194 + 236 +47.2
=477.2 m, which is within approximately 1% of the value derived from
Equation 6.6.
It is imperative that, in the interests of safety, along a given stretch of highway
there is no confusion on the driver’s part as to whether or not it is safe to over-
take. On stretches where overtaking is allowed, the minimum values given in166 Highway Engineering