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ECGD 4121 – Transportation Engineering I Lecture 8

Faculty of Applied Engineering and Urban Planning. Civil Engineering Department. 2 nd Semester 2008/2009. ECGD 4121 – Transportation Engineering I Lecture 8. Intersections Design & Control. Content. Intersections Interchanges Grade-separated intersections At-grade intersections

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ECGD 4121 – Transportation Engineering I Lecture 8

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  1. Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2nd Semester 2008/2009 ECGD 4121 – Transportation Engineering I Lecture 8

  2. Intersections Design & Control

  3. Content • Intersections • Interchanges • Grade-separated intersections • At-grade intersections • Channelization • Clearly defined travel paths for vehicles

  4. Interchanges • Ramps at different levels connecting two or more freeways

  5. Grade-separated Intersections • Two intersecting highways connected at different elevations

  6. At-grade Intersections • Two intersecting roads connected at the same elevation

  7. At-grade Intersections • Two intersecting roads connected at the same elevation

  8. Objectives of Traffic Channelization • To establish clear definition of vehicle paths • To control movement of traffic • To provide safe refuge for pedestrians • To separate traffic conflicts • To provide safe merging/diverging of traffic

  9. Channelized Intersections

  10. Intersections Control • Objective: to reduce the conflict points at an intersection • Method of control depends on: • Type of intersection (4-way, T-Intersection, etc.) • Traffic volume in each conflicting stream • Guidelines provided in Manual on Uniform Traffic Control Devices (MUTCD) • Guidelines presented in the form of warrants (i.e. conditions that call for intersection control)

  11. Intersection Control Using Yield Signs • Eliminates crossing conflicts • Used at major road-minor road intersections • Warrants: • Approach speed on minor road > 10 mph • When there is a separate channelized lane without adequate acceleration lane

  12. Intersection Control Using Stop Signs • Eliminates crossing & merging conflicts • Inconvenient to motorists • Used where approaching vehicle has to stop before the intersection • Warrants: • Minor road intersects with major road • Presence of hazardous conditions such as high approach speed, restricted view, and history of accidents

  13. Intersection Control Using Multi-Way Stop Signs • Used when traffic volumes on all approaches are approximately equal • For too high traffic volumes, use signals • Warrants: (satisfy all) • Total int. approach volume > 500 vph for any 8 hours of average day • Combined vehicle + pedestrian volume for minor approach > 200 units per hour for same 8-hr. period • Avg. vehicle delay on minor street > 30 sec/veh for max. hr • If 85th percentile approach speed on major approach is > 40 mph

  14. Intersection Control Using Traffic Signals • When minor road traffic suffers excessive delays • Minimum pedestrian volume: • Ped. Volume crossing major street > 100 for any 4 hrs • Ped. Volume crossing major street > 190 for any 1 hr • Traffic leaves less than 60 gaps accepted by pedestrians per hour • Nearest traffic signal > 300 ft. away • Use traffic actuated signal, push button operation for pedestrians

  15. Intersection Control Using Traffic Signals • School crossings • Used when traffic gaps are inadequate for safe crossing • When no. of accepted gaps < minutes in period • No parking within 100 ft. before and 20 ft. after crossing • Progressive movement • Exception to other warrants when it helps to maintain grouping of vehicles to regulate the group speed • Accident experience (when signal is suitable) • 5 or more injury in 12 months

  16. Intersection Control Using Traffic Signals • Peak hour delay: delay and volume in any 4 consec. 15-min periods when controlled by stop sign is: • > 4 veh-hrs & 100 vph for 2-lane minor street approach • > 5 veh-hrs & 150 vph for 2-lane minor street approach

  17. Signal Timing - Terminology • Controller - fixed or variable timing • Cycle (one complete color sequence) • Phase-part of cycle allocated to a stream of traffic • Interval - part of cycle when indications do not change • Offset - time lapse between green @ successive intersections • Change & clearance interval - Total time in seconds for yellow & all-red signal indications • All-red interval - when display is red for all directions

  18. Two-Phase Signal • Phase (A) • East-West Through Movement has Right-of-Way • Phase (B) • North-South Through Movement has Right-of-Way

  19. Two-Phase Signal: Phase (A)

  20. Two-Phase Signal: Phase (B)

  21. Four-Phase Signal • Phase A • East-West Left Turn Movement has Right-of-Way • Phase B • East-West Through Movement has Right-of-Way • Phase C • North-South Left Turn Movement has Right-of-Way • Phase D • North-South Through Movement has Right-of-Way

  22. Four-Phase Signal: Phase (A)

  23. Four-Phase Signal: Phase (B)

  24. Four-Phase Signal: Phase (C)

  25. Four-Phase Signal: Phase (D)

  26. Reduction in ConflictsDue to Traffic Signal No Signal2-Phase4-Phase Crossing Conflicts 16 4 0 Merge Conflicts 8 8 8 Diverging Conflicts 8 8 4

  27. Example 1 A section of a major highway has a speed-flow relationship of the form: q = au2 + bu It is known that the capacity is 2925 veh/hr and the corresponding space-mean speed of traffic is 30 mph. Determine the speed when the flow is 1400 veh/hr and the free-flow speed. 

  28. Example 1 - Solution @ qm = 2925 veh/hr, um = 30 mph @ qm, dq/du = 0 dq/du = 2aum + b = 0 (2)(a)(30) + b = 0 ……………... [1] qm = aum2 + bu 2925 = (a)(30)2 + (b)(30) …….. [2]

  29. Example 1 - Solution Solving for a & b  a = -3.25 & b = 195.00  q = -3.25u2 + 195u @ q = 1400 veh/hr, 1400 = -3.25u2 + 195u Solving for u  u1 = 8 veh/hr & u2 = 52

  30. Example 1 - Solution @ uf, q = 0  0 = -3.25uf2 + 195uf Solving for uf uf = 60 veh/hr OR uf = 2um = (2)(30) = 60 veh/hr

  31. Example 2 A section of a major highway has the following flow-density relationship: q = 42k-0.56k2 What is the capacity of the highway section, and the speed when the highway is at one quarter of its capacity.

  32. Example 2- Solution @ Capacity, q = qm , k = km, and u = um @ qm, dq/du = 0 dq/du = 42 - 1.12km = 0  km = 42/1.12 = 37.50 veh./mile Capacity = qm = 42km – 0.56km2  Capacity = qm = 787.5 veh./hr.

  33. Example 2- Solution @ One-quarter of capacity, q = qm /4 = 196.875 veh./hr. q = 196.875 = 42k – 0.56k2 0.56k2 - 42k + 196.875 = 0 Solving for k values,  k1 = 5 veh./mile OR k2 = 70 veh./mile

  34. Example 2- Solution @ q = 196.875 veh./hr. & k1 = 5 veh./mile, Speed = u1 = q/k1 = 39.375 mph OR @ q = 196.875 veh./hr. & k2 = 70 veh./mile, Speed = u2 = q/k2 = 2.8125 mph

  35. Quiz # 6 • Discuss briefly all the cases of traffic delay • Discuss briefly all the types of traffic volume counts

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