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

Intersections Design & Control

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

Interchanges Ramps at different levels connecting two or more freeways

Grade-separated Intersections Two intersecting highways connected at different elevations

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

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

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

Channelized Intersections

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)

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

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

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

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

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

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

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

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

Two-Phase Signal: Phase (A)

Two-Phase Signal: Phase (B)

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

Four-Phase Signal: Phase (A)

Four-Phase Signal: Phase (B)

Four-Phase Signal: Phase (C)

Four-Phase Signal: Phase (D)

Reduction in Conflicts Due to Traffic Signal No Signal 2-Phase 4-Phase Crossing Conflicts 16 4 0 Merge Conflicts 8 8 8 Diverging Conflicts 8 8 4

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. 

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]

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

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

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.

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.

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

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