Traffic Engineering – Street Segment Interrupted Flow

Street Segment Interrupted Flow is traffic flow regulated by an external means such as a traffic signal.

Vehicle-vehicle interactions and vehicle-roadway interactions play a secondary role in defining the traffic flow under interrupted flow conditions.

Concepts and Equations

From Signal Timing Tutorial:

• Critical Lane Volume

Maximum volume per lane during a signal phase.

• Critical Volume (V_c)

Sum of all critical lane volumes during a signal cycle.

• Through Car Equivalent Units (TCUS)

A hypothetical vehicle generated from vehicle types, vehicle turning movements, etc. for use in calculating traffic signal cycles.

TCUS are generated by multiplying traffic volumes by tabulated factors to account for the added congestion created by vehicles turning left or right from the flow of traffic.

• Saturation Flow Rate (s)

Number of vehicles served by a lane for one hour of green time (vehicles/hr).

The maximum number of vehicles that can be serviced in an hour of green is: s = 3600 ÷ h.

• Saturation Headway

Headway of the vehicles in a “stable moving platoon” passing through a green light.

• Lost Time

Elapsed time when no cars are present in an intersection. Equal to Start-Up Lost Time and Clearance Lost Time.

• Start-Up Lost Time

Sum of time difference between the saturation headway and the first four headways.

• Clearance Lost Time

Elapsed time required during right-of-way changes.

• Current Flow Rate (v)

Obtained from traffic counts.

• Desired v/c Ratio

Ratio of current flow rate to capacity of the facility. Equal to Rate of Flow divided by Capacity.

• Peak Hour Factor (PHF)

Calculated by: Hourly Volume divided by Maximum Rate of Flow.

Also, PHF = V ÷ (V₁₅ x 4)

• • where:
    • V = Peak Hourly Volume (vehicles/hr)
    • V₁₅ = maximum 15-minute volume within the peak hour

• Cycle Length (C)

Amount of time from when a movement is given the right-of-way until that movement receives it again.

C = (N x t_L) ÷ (1 – (V_c ÷ (PHF x (v/c)_des x 3600/h)))

• • where:
    • N = the number of phases in one cycle
    • t_L = total lost time per phase (sec)
    • V_c = Critical Volume (vehicles/hr)
    • PHF = Peak Hour Factor
    • (v/c)_des = desired volume/capacity ratio
    • h = saturation headway (sec)

• Green Time Calculations

  • Green Split – how long each phase will have the right-of-way.

  • Effective Green Time (g_i) – time that a movement is going, regardless of the indication shown.

g_i = (V_ci ÷ V_c) x (C – L)

• • • where:
      • g_i = Effective Green Time (sec)
      • V_ci = Critical volume for phase (vehicles/in-hr)
      • V_c = Total Critical Volume (vehicles/in-hr)
      • C = Cycle length (sec)
      • L = Total Lost Time (t_L x N) (sec)

• Actual Green Time (G_i)

G_i = g_i + t_L – Y_i

• • where:
    • t_L = Lost time per phase (sec)
    • Y_i = Yellow and all-red time (sec)

• Intersection Capacity (c_i)

c_i = (3600/h) x (g_i/C) = s(g_i/C)

• • where:
    • c_i = Capacity of one approach lane (passenger cars per lane per hour)
    • h = Saturation Headway
    • g_i = Effective Green Time for the approach (sec)
    • C = Cycle length (sec)
    • s = Saturation Flow Rate = 3600/h

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Related Content:

• Traffic Flow
• Introduction to Traffic Flow
• Traffic Flow Characteristics
• Introduction to Transportation Engineering
• Handbook of Transportation Engineering
  • Chapter 1 – Traffic Engineering Analysis
  • Chapter 3 – Traffic Congestion
  • Chapter 4 – Highway Capacity
    • 4.5 – The Capacity of Interrupted Flow Facilities
  • Chapter 6 – Traffic Signals
  • Chapter 10 – Intersection and Interchange Design
• Theory of Traffic Flow – Statistical Models
• Traffic Volume Estimations

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To Your Success …

Jeff Setzer, PE

Street Segment Interrupted Flow

NCEES