What is the approximate psychological widening required for a pavement of 7 m on a horizontal curve of radius 225 m, if the longest wheel base of vehicle expected on the road is 7·0 m ? (Take the design speed as 70 km/h)
- A0·40 m
- B0·50 mCorrect
- C0·60 m
- D0·70 m
Solution
Concept
Psychological widening (IRC)
Formula
Wps = V / (9.5 √R)
Step-by-step solution
1. V = 70 km/h, R = 225 m, √R = 15. 2. Wps = 70 / (9.5 × 15) = 0.49 m ≈ 0.50 m.
Explanation
Psychological widening is the extra width drivers need for comfortable clearance at higher speed on curves. It depends on the speed and radius, not on the wheel base (which affects mechanical widening).
Why the other options are wrong
0.40, 0.60 and 0.70 m do not equal V/(9.5√R) = 0.49 m.
Shortcut
Wps = V/(9.5√R); Wmech = n l²/(2R).
Common mistake
Using the mechanical widening formula with the 7 m wheel base.
Correct answer: B) 0·50 m