Traffic Signal Design - Examples & Applications

Example: Webster's Method for Signal Timing

Let's design a simple two-phase pre-timed signal using Webster's Method.

Example

Problem Statement: A simple cross intersection has a two-phase signal.

Phase 1 (North-South): Design Flow ( $v_1$ ) = 800 veh/hr. The approach has a calculated saturation flow ( $s_1$ ) = 2,000 veh/hr.
Phase 2 (East-West): Design Flow ( $v_2$ ) = 600 veh/hr. The approach has a calculated saturation flow ( $s_2$ ) = 1,800 veh/hr.
The lost time per phase ( $l_i$ ) is estimated at 3 seconds.

Calculate the optimum cycle length and the effective green time allocated to each phase.

Solution: Webster's Method Calculation

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Example: Green Time Allocation

After determining the cycle length, the effective green time must be distributed among the phases proportional to their flow ratios.

Example

Problem Statement: Following Webster's method, an intersection has a cycle length (

C

) of 70 seconds and a total lost time (

L

) of 8 seconds. The flow ratios are

y_1 = 0.35

and

y_2 = 0.40

(Total

Y = 0.75

). Calculate the effective green time for Phase 1 (

g_1

) and Phase 2 (

g_2

Given:

$C = 70$ , $L = 8$
$y_1 = 0.35$ , $y_2 = 0.40$ , $Y = 0.75$

Step-by-Step Solution

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Example: Critical Lane Volume

Identifying the critical lane groups is the first step in signal timing.

Example

Problem Statement: A North-South street has two phases. Phase A serves through and right-turn traffic (Volume: 450 vph in lane 1, 500 vph in lane 2). Phase B serves an exclusive left-turn lane (Volume: 150 vph). Determine the critical lane volume (

v_c

) for the North-South approach.

Given:

Phase A lane volumes: 450 vph, 500 vph
Phase B lane volume: 150 vph

Step-by-Step Solution

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Example: Pedestrian Clearance Time

Signals must provide adequate time for pedestrians to cross safely.

Example

Problem Statement: A crosswalk is 60 ft long. The standard pedestrian walking speed is 3.5 ft/s. Determine the required pedestrian clearance time (Flashing Don't Walk interval).

Given:

Crosswalk length ( $D$ ) = 60 ft
Walking speed ( $S_p$ ) = 3.5 ft/s

Step-by-Step Solution

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