Lectures 12 to 13 (Chapters 23) Analyses of Unsignalized Intersections

1. Lectures 12 to 13 (Chapters 23) Analyses of Unsignalized Intersections

2. Types of Unsignalized Intersections • Uncontrolled • Stop Sign Controlled • TWSC • AWSC • Yield Sign Controlled • Roundabout

3. TWSC Intersections • Priority movements

4. Gap Acceptance • Critical gap, tc • Minimum gap in the major stream that a minor street driver can accept • Cannot be measured directly in the field • Drivers behave differently • Follow-up time, tf • Minimum headway between two consecutive vehicles in the minor stream • Can be measured directly in the field Veh1 Veh2 tf tmv

5. tc and tf

6. Adjustments on tc and tf

7. Capacity • Conflicting volume, vcx • Figure 23.3 (page 670) • Potential capacity, cpx • Movement capacity, cmx For minor street LT at T intersection

8. Delay and LOS * (xx ) for signalized intersections

9. Exercise Using HCS to solve the sample problem given. After the model is set up, try to answer the following questions. • Can you duplicate the results given in the example? • How many minor movements are involved in the example? • Which minor movements have the same potential capacity and movement capacity, i.e., which minor movements are not impeded by higher priority minor movements? • What is the basic critical gap and final critical gap for the minor street LT movement? • What is the conflicting flow rate for minor street LT movement? Verify the result manually. • What is the probability of major street LT movement not blocking the minor street LT movement (i.e., probability of queue free for major street LT movement)? • what is the impedance factor for minor street LT movement? How is this factor used for the calculation? • What are the movement capacities for the minor street LT and RT movements? • What is the capacity for the minor street approach (shared LT and RT lane) • What is the control delay for the minor street approach?

10. Complexities • * Upstream Signal Effect • Two-stage crossing • Flared approach

11. Roundabout • Roundabout is a special type of traffic circle • Major differences between roundabout and traffic circle • Speed • Traffic control • Priority of circulating traffic • Design vehicle • Pedestrian access • Parking • Direction of circulation • Types of roundabout • Mini • Urban: compact, single-lane, double-lane • Rural: single-lane, double-lane

12. Roundabout Design Elements Inscribed circle diameter Exit width Entry width Splitter island width Circulatory roadway width

13. Roundabout Operations - HCM • Roundabouts introduced in 1997 HCM • Method unchanged in HCM 2000 • Uses analytical method • Only estimates capacity; no guidance on delay or level of service • Measured in PCE

14. vc va FHWA Method • Consideration of vehicle types (cars, trucks, motorcycles) • Empirical regression model • Delay and LOS http://www.tfhrc.gov/safety/00068.pdf

15. FHWA Method vs. HCM FHWA HCM

16. Steps to Calculate Roundabout Volumes Turning movements Roundabout volumes

17. Steps • Step 1: Convert trucks and other vehicle types to passenger car equivalents (pce) • Step 2: PHF volume adjustment • Step 3: Entry volume • Step 4: Exit volume • Step 5: Circulating volume

18. Entry Volume • Entry volume = sum of entering turning movements

19. Exit Volume • Exit volume = sum of turning movements as shown

20. Circulating Volume • Circulating volume = sum of turning movements as shown

21. Example: Volume Conversion Turning movements Roundabout volumes PHF = 0.94 East/west:2% SU trucks/ buses 70 530 120 ? 50 240 100 110 350 140 80 410 90 North/south:4% SU trucks/buses, 2% combo trucks

22. Example Step 1: PCE Calculation • SB TH: 530 veh (4% SU/bus, 2% combo) % cars (0.94) 1.0 pce/veh = 0.94 % SU/bus (0.04) 1.5 pce/veh = 0.06 % combo (0.02) 2.0 pce/veh = 0.04 fhv = 1.04  530 veh 551 pce

23. Example Step 1: Completed PCE Calculation Raw Counts PCEs 70 530 120 73 551 125 50 240 100 51 242 101 110 350 140 111 354 141 80 410 90 83 426 94

24. 73 551 125 51 242 101 111 354 141 83 426 94 Example Step 2: PHF Factor PCEs (peak 15 minutes) PCEs (hourly) 78 586 133 54 258 107 118 377 150 88 454 100 141 / 0.94 = 150

25. Example Step 3: Calculate Entry Volume • Entry volume = sum of entering turning movements 797 78 586 133 ? ? ? ? 54 258 107 419 118 377 150 645 ? ? ? ? 88 454 100 642

26. Example Step 4: Calculate Exit Volume • Exit volume = sum of turning movements as shown 797 78 586 133 626 ? 424 ? 54 258 107 419 118 377 150 645 ? 610 ? 843 88 454 100 642

27. Example Step 5: Calculate Circulating Volume • Circulating volume = sum of turning movements as shown 797 78 586 133 626 453 424 826 54 258 107 419 118 377 150 645 660 610 628 843 88 454 100 642

28. Example: Solution Turning movements Roundabout volumes 797 PHF = 0.94 East/west:2% SU trucks/ buses 70 530 120 626 453 424 826 50 240 100 419 110 350 140 645 660 610 628 843 80 410 90 North/south:4% SU trucks/buses, 2% combo trucks 642

29. Example: Capacity Calculation Roundabout volumes Roundabout category? 797 626 453 ? 424 826 419 645 660 610 628 843 642

30. Example: Capacity Calculation 424 826 Check capacity of each entry 645

31. 1400 1200 1000 Entering and circulating flow = 1800 veh/h 762 800 Maximum Entry Flow (veh/h) 600 607 400 Urban & Rural Single-Lane Roundabouts Urban Compact Roundabouts 200 0 826 0 400 800 1200 1600 2000 2400 Circulatory Flow (veh/h) Example: Capacity check

32. Example: Capacity Check • Entering volume = 645 pce/h • Capacity of entry: • Single-lane: 762 pce/h • Urban compact: 607 pce/h • Volume-to-capacity ratio: • Single-lane: 645 / 762 = 0.85 AT • Urban compact: 645 / 607 = 1.06 OVER • What to do when v/c >1.0? • Repeat calculation for other approaches What to do when v/c >1.0?

33. Solution: Capacity Calculation Urban Compact Single-Lane 0.90 0.83 OVER CAPACITY OK (TODAY) 0.57 0.49 1.06 0.85 0.85 0.74

34. Delay • Control delay • Includes initial deceleration delay, queue move-up time, stopped delay, and final acceleration delay • Geometric delay • Delay experienced by a single vehicle with no conflicting flows • Caused by geometric features • Total delay = Control + Geometric • Typical measure used: control delay