1. ECGD 4121 – Transportation Engineering I
Faculty of Applied Engineering and Urban Planning
Civil Engineering Department
1st Semester 2009/2010
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 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 Conflicts Due to Traffic Signal
No Signal 2-Phase 4-Phase
Crossing Conflicts
Merge Conflicts
Diverging Conflicts

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 = -3.25u u
@ q = 1400 veh/hr,
1400 = -3.25u u
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 = km = 0
 km = 42/1.12 = veh./mile
Capacity = qm = 42km – 0.56km2
 Capacity = qm = veh./hr.

33. Example 2- Solution @ One-quarter of capacity,
q = qm /4 = veh./hr.
q = = 42k – 0.56k2
0.56k2 - 42k = 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 = mph OR
@ q = veh./hr. & k2 = 70 veh./mile,
Speed = u2 = q/k2 = mph