Test 19 - Traffic Signal Design by Webster Method

Question 1 ప్రశ్న 1

For an isolated fixed-time signalized intersection operating within the usual range of Webster’s method, which expression gives the approximate minimum-delay cycle length C0 in seconds when L is the total lost time per cycle and Y is the sum of critical flow ratios?

A C0 = (L + 5)/(1 + Y). B C0 = (1.5L + 5)/(1 - Y). C C0 = (1.5Y + 5)/(1 - L). D C0 = L/(1 - Y), without the Webster correction terms.

Question 2 ప్రశ్న 2

For a two-phase signal, each phase serves two compatible lane groups concurrently without shared-lane constraints. How should the critical flow ratio for each phase generally be identified for a basic Webster calculation?

A Add the four lane-group demands and divide by the sum of all four saturation flows. B Add the flow ratios of all compatible lane groups within each phase. C Select the lane group with the smallest absolute traffic volume in each phase. D Use the largest demand-to-saturation-flow ratio among lane groups served concurrently in each phase, then sum the two phase-critical ratios.

Question 3 ప్రశ్న 3

A signalized traffic lane has an observed saturation discharge headway of 2.25 seconds per vehicle after start-up. What is its corresponding saturation flow rate, assuming the headway remains uniform?

A 1600 vehicles/h/lane. B 1350 vehicles/h/lane. C 1800 vehicles/h/lane. D 2250 vehicles/h/lane.

Question 4 ప్రశ్న 4

Two compatible lane groups are served simultaneously during one phase, with no shared-lane restrictions. Group P carries 540 vehicles/h at a saturation flow of 1800 vehicles/h, and group Q carries 400 vehicles/h at a saturation flow of 1600 vehicles/h. What is the phase-critical flow ratio?

A 0.25. B 0.55. C 0.30. D 0.75.

Question 5 ప్రశ్న 5

A three-phase Webster analysis gives phase-critical flow ratios of 0.25, 0.30 and 0.20 for three sequential phases. What is the sum of critical flow ratios Y?

A 0.55. B 0.75. C 0.85. D 1.25.

Question 6 ప్రశ్న 6

A signal has three sequential phases, each with 2 seconds of start-up lost time and 2 seconds of clearance lost time. What is the total lost time L per cycle for the adopted Webster calculation?

A 12 seconds. B 6 seconds. C 8 seconds. D 18 seconds.

Question 7 ప్రశ్న 7

Using Webster’s expression C0 = (1.5L + 5)/(1 - Y), determine the optimum cycle length for L = 10 seconds and Y = 0.60.

A 40 seconds. B 45 seconds. C 60 seconds. D 50 seconds.

Question 8 ప్రశ్న 8

A proposed signal has total lost time L = 12 seconds and sum of critical flow ratios Y = 0.70. Under Webster’s formula, what is the approximate optimum cycle length to one decimal place?

A 66.7 seconds. B 70.0 seconds. C 76.7 seconds. D 86.7 seconds.

Question 9 ప్రశ్న 9

A three-phase fixed-time signal has critical lane-group flows of 300, 450 and 750 vehicles/h, with a saturation flow of 2000 vehicles/h for each critical group. Each phase has 4 seconds of lost time. Applying Webster’s optimum-cycle formula and splitting effective green in proportion to the critical flow ratios, which complete design result is correct?

A C0 = 92 seconds; effective greens = 16, 24 and 40 seconds. B C0 = 92 seconds; effective greens = 20, 30 and 50 seconds. C C0 = 80 seconds; effective greens = 16, 24 and 40 seconds. D C0 = 92 seconds; effective greens = 40, 24 and 16 seconds.

Question 10 ప్రశ్న 10

A fixed-time signal has an adopted cycle length of 84 seconds and a total effective lost time of 12 seconds per cycle. Under the usual sequential-phase timing convention, how much total effective green time is available?

A 84 seconds. B 72 seconds. C 96 seconds. D 60 seconds.

Question 11 ప్రశ్న 11

A two-phase signal has an adopted, not necessarily Webster-optimum, cycle length of 60 seconds, lost time L = 10 seconds and critical flow ratios y1 = 0.24 and y2 = 0.36. Using proportional Webster green splitting, what are the two effective green times?

A 24 seconds and 36 seconds. B 25 seconds and 25 seconds. C 30 seconds and 20 seconds. D 20 seconds and 30 seconds.

Question 12 ప్రశ్న 12

A three-phase signal has an adopted cycle of 80 seconds, total lost time of 8 seconds and critical ratios of 0.15, 0.25 and 0.20. What are the proportional effective green allocations, in phase order?

A 15, 25 and 20 seconds. B 20, 32 and 28 seconds. C 18, 30 and 24 seconds. D 24, 30 and 18 seconds.

Question 13 ప్రశ్న 13

For a signal phase, effective green g = 24 seconds, amber Yc = 3 seconds, all-red R = 1 second, start-up lost time l1 = 2 seconds and clearance lost time l2 = 3 seconds. Using g = G + Yc + R - l1 - l2, what displayed green G is required?

A 23 seconds. B 25 seconds. C 24 seconds. D 29 seconds.

Question 14 ప్రశ్న 14

For one phase the start-up lost time is 2.5 seconds and clearance lost time is 1.5 seconds. Its amber indication lasts 3 seconds and its all-red indication lasts 2 seconds. What phase lost time should be included in Webster’s total L using the stated lost-time convention?

A 4.0 seconds. B 5.0 seconds. C 6.5 seconds. D 9.0 seconds.

Question 15 ప్రశ్న 15

For a signal with fixed total lost time L = 10 seconds, Webster’s expression is used first with Y = 0.50 and then with Y = 0.75. What are the respective optimum cycles?

A 50 seconds and 75 seconds. B 40 seconds and 60 seconds. C 30 seconds and 60 seconds. D 40 seconds and 80 seconds.

Question 16 ప్రశ్న 16

A proposed isolated fixed-time intersection has Y = 1.02 and a positive total lost time L. Which engineering interpretation of Webster’s optimum-cycle expression is most defensible?

A Use a negative cycle because the formula denominator is negative. B Do not treat the resulting nonphysical cycle as a valid design; review demand, capacity, geometry or phasing because Y exceeds unity. C Set total lost time to zero and retain all demand without any further analysis. D Use the numerator 1.5L + 5 as the optimum cycle regardless of Y.

Question 17 ప్రశ్న 17

For a signal with Y = 0.75, total lost time L = 12 seconds and adopted cycle C = 60 seconds, compute the illustrative critical degree-of-saturation measure X = Y/[1 - L/C].

A 0.75. B 0.80. C Approximately 0.94. D 1.25.

Question 18 ప్రశ్న 18

A signalized lane group has saturation flow s = 1800 vehicles/h, allocated effective green g = 24 seconds and cycle length C = 60 seconds. If its demand is 600 vehicles/h, which capacity and volume-to-capacity ratio are correct using c = s(g/C)?

A Capacity 720 vehicles/h and v/c approximately 0.83. B Capacity 600 vehicles/h and v/c exactly 1.00. C Capacity 1800 vehicles/h and v/c approximately 0.33. D Capacity 900 vehicles/h and v/c approximately 0.67.

Question 19 ప్రశ్న 19

For sequential phases, assume the available effective green is C - L and phase greens can be allocated in the proportions y_i/Y. If total lost time is L = 12 seconds and critical sum Y = 0.75, what is the minimum theoretical cycle length ensuring the summed critical demand does not exceed the available effective-green share?

A 36 seconds. B 40 seconds. C 48 seconds. D 92 seconds.

Question 20 ప్రశ్న 20

Two compatible approaches can operate concurrently in a proposed signal phase without opposing or pedestrian conflicts. Their critical flow ratios are 0.22 and 0.31. Which treatment is appropriate when constructing a basic Webster critical-path analysis for that shared phase?

A Add both ratios to obtain 0.53 because every separate approach needs its own whole phase. B Average the ratios to obtain 0.265 without checking shared saturation constraints. C Exclude both ratios because compatible approaches need no green. D Use the governing concurrent critical ratio of 0.31, subject to verifying the actual lane-group and movement compatibility.

Question 21 ప్రశ్న 21

Webster’s calculation produces a phase-effective green that is shorter than the required pedestrian crossing service time for a movement scheduled with that phase. Which design response is appropriate?

A Apply the pedestrian minimum service constraint and revise the green split or cycle as needed, then recheck the whole timing plan. B Use the shorter computed green without change because Webster’s formula overrides pedestrian requirements. C Remove the pedestrian movement from the intersection capacity study and leave its signals unchanged. D Reduce the prescribed pedestrian clearance time solely to make the computed split fit.

Question 22 ప్రశ్న 22

Two sequential signal phases have critical lane-group flows and saturation flows as follows: Phase 1 has 450 and 1500 vehicles/h; Phase 2 has 480 and 2400 vehicles/h. What is the correct sum of critical flow ratios Y?

A 0.238. B 0.40. C 0.50. D 0.80.

Question 23 ప్రశ్న 23

A two-phase signal has start-up lost times of 2.0 and 1.5 seconds and clearance lost times of 2.5 and 2.0 seconds, respectively. What total cycle lost time L should be used in Webster’s expression?

A 6.0 seconds. B 8.0 seconds. C 9.5 seconds. D 12.0 seconds.

Question 24 ప్రశ్న 24

A two-phase signal has phase-critical flows of 450 and 630 vehicles/h, each with saturation flow of 1800 vehicles/h. The total lost time is 10 seconds per cycle. Using Webster’s optimum-cycle expression followed by proportional effective-green allocation, which is the correct result to one decimal place?

A C0 = 60 seconds; g1 = 20.0 seconds and g2 = 30.0 seconds. B C0 = 50 seconds; g1 = 20.0 seconds and g2 = 20.0 seconds. C C0 = 50 seconds; g1 = 23.3 seconds and g2 = 16.7 seconds. D C0 = 50 seconds; g1 = 16.7 seconds and g2 = 23.3 seconds.

Question 25 ప్రశ్న 25

A Webster calculation for a heavily loaded isolated junction gives a very long optimum cycle, while the site also has pedestrian crossing requirements, short turning-bay storage and coordination constraints. Which engineering judgement is most appropriate?

A Treat the Webster cycle as a preliminary estimate and compare feasible timings and geometric or operational alternatives against capacity, queueing, pedestrian safety and coordination needs. B Adopt the unrounded Webster cycle regardless of spillback and pedestrian clearance requirements. C Assume that extending the cycle indefinitely always reduces average delay as Y approaches one. D Use only the numerical cycle length and omit checks of lane-group saturation flow, storage and lost time.

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