Junction Modelling in a Strategic Transport Model

1. Junction Modelling in a Strategic Transport Model Wee Liang Lim Henry Le Land Transport Authority, Singapore

2. Outline • Background • Objective • Overview of the LTA Strategic Transport Model • Review of iterative junction modelling • Revised junction modelling • Comparison of performance results • Conclusions

3. Background Singapore • A city state • 648 km2 area ; 4.1 mil population. • 109 km rail lines (MRT/LRT), 150 km expressways • 575 km major arterial roads, 1500 signalised junctions EMME/2 Strategic Transport Model • Used widely to forecast travel demand for planning & design of transport proposals, also calculate user benefits • Enhanced over the years • Incorporated “iterative” junction modelling in 2000 • Recently revised junction modelling

4. Objective • To present a review of the iterative approach in junction modelling and its limitations. • To present a revised & simpler approach in junction modelling and its improvements in model convergence

5. OVERVIEW OF LTA STRATEGIC TRANSPORT MODEL Model Outputs Model Inputs Model Step Land use data Daily trip ends by purpose - Planning Data: population, employment, school enrolment. - Car ownership, - Dwelling types & others HBW, HBS, HBB HBL, NHB Trip Generation Trip rate data Trip distribution matrices by trip purpose and main mode Trip distribution functions HBW (highway, transit) HBS, HBB, HBL, NHB Trip Distribution HIS data Skims of time and cost Daily OD matrices by mode and trip purpose From assignments - Car, m/c, Taxi - LRT/MRT/Bus HBW (car, m/c, taxi, LRT, MRT, bus, c/o bus) HBS (car, LRT, MRT, bus, school bus) HBB, HBL, NHB Mode Split Mode split parameters From HIS and SP survey Peak hour matrices, AM, PM & OP by mode Peak hour factors by trip purpose, mode and area Peak Hour Factors - car, m/c, taxi - LRT/MRT - c/o bus, school bus - bus From HIS and traffic count data Special trip matrices - tourist trips, airport trips - goods vehicle trips Model outputs - travel times - highway volumes - transit volumes - other performance measures for downstream analysis (e.g. financial, economic analysis) Network Trip Assignment - links, junctions - travel time, delay functions - transit services iteration

6. Standard Iterative Approach Start Start Calculate movement capacity & effective green time Calculate link delay Calculate link delay Calculate Junction delay Assign Traffic Run assignment for N iterations Check Convergence No Check Convergence No Yes Yes END END Junction Modelling - Iterative Approach Review Assignment Procedure

7. Iterative Approach Review Junction Coding • Turn penalty (delay) function (tpf): • User defined turn data • UP1: 6 digits to store 1: No. of lanes 2: No. of short lanes 3: Shared lane description 4: Signal control or not 5: Opposed information 6: unused • UP2: unopposed green time & opposed green time • UP3: cycle time • Extra user turn data: effective green time & capacity

8. Iterative Approach Review Delay Function for Signalised Movement • Delay function was based on SIDRA Formulae • Delay = uniform delay + Overflow delay • Function of cycle time, green split, arrival flow and movement capacity D(delay) = c/2*(1-u)2/(1-u*x) + 900*(x-1 + Sqr((x-1)2 + 4x/C))

9. Iterative Approach Review Movement Capacity • Unopposed Movement • Capacity = Saturation flow*green time/cycle time • Opposed Movement: • Opposing movement & flow • Effective saturation flow • Effective capacity for opposed movement • Movement in a shared lane: • Capacity is proportioned to the ratio of its flow over total lane flow.

10. 2nd Iteration 4th Iteration 1st Iteration 3rd Iteration Iterative Approach Review Limitations Assignment & convergence instability. Factors identified: (i) Steep junction delay curve (ii) Iterative calculation of movement capacity

11. REVISED JUNCTION MODELLING

12. Revised Approach Objectives • To represent realistically the junction delay in a strategic network • To improve model convergence and therefore assignment stability and accuracy

13. Iterative Approach Start Calculate movement capacity & effective green time Calculate link delay Calculate Junction delay Assign Traffic Check Convergence No Yes END Junction Modelling - Revised Approach Assignment Procedure Revised Approach Start Calculate movement capacity & effective green time Calculate link delay Calculate Junction delay Assign Traffic Check Convergence No Yes END

14. Revised Approach Revised Delay Function To reduce the steep gradient of the iterative delay curve Delay = {0.25 + 0.25 (V/C)}*{c-g} for V/C <1 {0.5 + 1.5 (V/C-1)}*{c-g} 1 < V/C < 2 {2 + 2 (V/C - 2)}* {c-g} 2 < V/C Source: V/C < 1: uniform delay V/C > 1: calibration of the base model

15. Revised Approach Revised & Improved Calculation of Movement Capacity • Different base saturation flow (veh/hour) Left Through Right 1700 1960 1800 • Simplified calculation for shared lane movements Saturation flow = base saturation flow/no. movements • Added calculation for short Lane Saturation flow = storage length/(vehicle space* mov. green time) (Capacity  400 veh/hr) • Simplified calculation for opposed movement Saturation flow = base saturation flow/3 (Capacity  200 veh/hr)

16. COMPARISON OF PERFORMANCE RESULTS

17. Comparison of movement delays Left Movement Iterative: Ave 16.8 sec Revised: Ave 22.2 sec 32% increase

18. Comparison of movement delays Through Movement Iterative: Ave 30.0 sec Revised: Ave 27.0 sec 10% reduction

19. Comparison of movement delays Right Movement Iterative: Ave 38.4 sec Revised: Ave 43.2 sec 12.5 % Increase

20. Comparison of network travel time 1999 Network - AM peak • Observations: • Junction delay increased despite delay curve smoothened • Link travel time reduction => more efficient route choice, more converged assignment

21. Comparison between modelled and observed traffic volumes

22. Comparison between modelled and observed travel time

23. Improvement in model convergence Comparison of model running time on the 2015 network Note: (38) number of iterations per highway assignment The revised approach has improved model convergence through reducing number of iterations & running time.

24. Conclusion • Junction delay is a major contributor to a journey time in an urban network. • Full incorporation of SIDRA to a strategic transport model may not suitable. • Revised and simpler approach to calculation of junction delay was presented • The revised model represents realistic movement delays, travel times and traffic demand in a network. • Model converges faster and predicts stable travel time & saving for transport schemes.