Module 2: Geometric design of highways

Lecture 13 Sight distance

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Problems

1. Calculate SSD for $V$=50kmph for (a) two-way traffic in a two lane road (b) two-way traffic in single lane road. (Hint: f=0.37, t=2.5) [Ans: (a)61.4 m (b) 122.8 m.

   Given: $V$=50km/hr = 13.9m/s $f$=0.37 $t$= 2.5 sec stopping distance=lag distance $+$ braking distance

   $$\begin{eqnarray*} \nonumber SD= vt +v^2/2gf \end{eqnarray*}$$
   
   
   
   Stopping Distance = 61.4 m
   Stopping sight distance when there are two lanes = stopping distance= 61.4m.
   Stopping sight distance for a two way traffic for a single lane = 2[stopping distance]=122.8m
   
   
2. Find minimum sight distance to avoid head-on collision of two cars approaching at 90 kmph and 60 kmph. Given t=2.5sec, f=0.7 and brake efficiency of 50 percent in either case. (Hint: brake efficiency reduces the coefficient of friction by 50 percent). [Ans: SD=153.6+82.2=235.8m]

   Given: $V_1$ =90 Km/hr. $V_2$ = 60 Km/hr. $t$ = 2.5sec. Braking efficiency=50%. $f$=.7.
   Stopping distance for one of the cars
   

   $$\begin{eqnarray*} \nonumber SD= vt +v^2/2gf \end{eqnarray*}$$
   
   
   
   Coefficient of friction due to braking efficiency of 50% = 0.5*0.7=0.35. Stopping sight distance of first car= $SD_1$= 153.6m
   Stopping sight distance of second car= $SD_2$= 82.2m
   Stopping sight distance to avoid head on collision of the two approaching cars $SD_1$+ $SD_2$=235.8m.
   
   
   

3. Find SSD for a descending gradient of 2% for V=80kmph. [Ans: 132m].

   Given: Gradient(n) = -2$V$ = 80 Km/hr.

   $$\begin{eqnarray*} \nonumber SD= vt +v^2/2g(f-n\%) \end{eqnarray*}$$
   
   
   
   SSD on road with gradient = 132m.
   
   
4. Find head light sight distance and intermediate sight distance for V=65 kmph. (Hint: f=0.36, t=2.5 s, HSD=SSD, ISD=2*SSD) [Ans: 91.4 and 182.8 m]

   Given: $V$=65km/hr $f$=0.36 $t$= 2.5 sec
   

   $$\begin{eqnarray*} \nonumber SD= vt +v^2/2gf \end{eqnarray*}$$
   
   
   
   Headlight Sight distance = 91.4m.
   Intermediate Sight distance= 2[SSD]= 182.8m.
5. Overtaking and overtaken vehicles are at 70 and 40 kmph respectively. find (i) OSD (ii) min. and desirable length of overtaking zone (iii) show the sketch of overtaking zone with location of sign post (hint: a=0.99 m/sec2) [Ans: (i) 278 m (ii) 834 m/1390]
6. Calculate OSD for V=96 kmph. Assume all other data. (Hint: Vb=96-16kmph. a=0.72, t=2.5s) [Ans: OSD one way 342m, OSD two way 646m]