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Understanding Platoon Dispersion in the TRANSYT-7F Traffic Flow Model

TRANSYT-7F, a U.S. adaptation of the UK TRANSYT model, incorporates both optimization and macroscopic simulation components to analyze traffic flow. A key aspect of this model is the Platoon Dispersion, which accounts for the dispersion of vehicle platoons originating from traffic signals over time and space. This dispersion leads to non-uniform vehicle arrivals at downstream signals, affecting vehicle delay calculations and overall signal effectiveness. By understanding this model, traffic engineers can improve timing and progression strategies at signalized intersections.

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Understanding Platoon Dispersion in the TRANSYT-7F Traffic Flow Model

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  1. Topic 5 Platoon and Dispersion

  2. TRANSYT-7F MODEL • TRANSYT is a computer traffic flow and signal timing model, originally developed in UK. • TRANSYT-7F is a U.S. version of the TRANSYT model, developed at U of Florida (Ken Courage) • TRANSYT-7F has an optimization component and a simulation component. • The simulation component is considered as a macroscopic traffic simulation, where vehicles are analyzed as groups. • One of the well known elements about TRANSYT-7F’s traffic flow model is the Platoon Dispersion model.

  3. WHY MODEL PLATOON DISPERSION? • Platoons originated at traffic signals disperse over time and space. • Platoon dispersion creates non-uniform vehicle arrivals at the downstream signal. • Non-uniform vehicle arrivals affect the calculation of vehicle delays at signalized intersections. • Effectiveness of signal timing and progression diminishes when platoons are fully dispersed (e.g., due to long signal spacing).

  4. PLATOON DISPERSION MODEL • For each time interval (step), t, the arrival flow at the downstream stopline (ignoring the presence of a queue) is found by solving the recursive equation

  5. PLATOON DISPERSION Flow rate at interval t, qt 100 % Saturation 50 0 Time, sec Start Green T = 0.8 * T’ Flow rate at interval t + T, Q(T+t) 100 % Saturation 50 0 Time, sec

  6. CLOSED-FORM PLATOON DISPERSION MODEL s Flow rate, vph v 0 tq tg Time C

  7. CLOSED-FORM PLATOON DISPERSION MODEL (1~tq) For 1~tq with s flow

  8. CLOSED-FORM PLATOON DISPERSION MODEL (0~tq) (1) (2) (1) – (2)

  9. CLOSED-FORM PLATOON DISPERSION MODEL (1~tq) For 1~tq with s flow Maximum flow downstream occurs at T+tq with upstream s flow

  10. BEYOND (1~tq) From the original equation: s no longer exists, but zero flow upstream t = tq +1 ~ ∞ • This is mainly to disperse the remaining flow, Qs,max. Upstream flow is zero • The same procedure for the non-platoon flow • The final will be the sum of the two

  11. EXAMPLE • Vehicles discharge from an upstream signalized intersection at the following flow profile. Predict the traffic flow profile at 880 ft downstream, assuming free-flow speed of 30 mph, α = 0.35; β = 0.8. Use time step = 1 sec/step 3600 Flow rate, vph 1200 0 16 28 Time C=60 sec

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