A fixed time 2-phase signal is to be provided at an intersection having four arms. The design hour traffic and saturation flow are as under: The time lost per phase due to starting delays is 2 seconds. Estimate the optimum signal cycle length.
- A40 secCorrect
- B45 sec
- C50 sec
- D55 sec
Solution
Concept
Webster
Formula
C₀ = (1.5L + 5)/(1 − Y)
Step-by-step solution
1. Phase 1: max(800/2400, 400/2000) = 0.333. 2. Phase 2: max(750/3000, 600/3000) = 0.25. 3. Y = 0.583; 4. an amber of 3 s per phase as lost time, lost time per phase = 2 + 3 = 5 s, Then L = 2 × 5 = 10 s. 5. Co = (1.5×10 + 5)/0.417 = 20/0.417 = 48 s. 6. Round up to a practical value: 50 s.
Explanation
Webster's method gives the optimum cycle length from the total lost time L and the sum of critical flow ratios Y.
Correct answer: A) 40 sec