Intersection Control Chapter 8 Dr. TALEB AL-ROUSAN

General Concepts of Traffic Control  Control Purpose: to assign the right of way to drivers, and thus to facilitate highway safety by ensuring the orderly and predictable movement of all traffic on highways.  Control is achieved by: Traffic signals Signs Markings  Those regulate, guide, warn, and/or channel traffic.  The more complex the maneuver area, the greater the need for a properly designed traffic control system.  See Manual of Uniform Traffic Control Devices (MUTCS) for guidelines for suitability of a particular control type.

Traffic Control Devices (TCD)  To be effective, Traffic control devices must: Fulfill a need. Command attention. Convey a clear simple meaning. Command the respect of road users. Give adequate time for proper response.

Traffic Control Devices (TCD)  To ensure that (TCD) possesses the five properties, MUTCD recommend that engineer consider the following five factors: Design: size, color, shape. Placement: within cone of vision of viewer and at distance to ensure proper response time. Operation: fulfill traffic requirements in a consistent and uniform way. Maintenance: regularly maintained to sustain legibility. Uniformity: to facilitate recognition and understanding by drivers, similar devices should be used at locations with similar traffic and geometric characteristics.

Conflict Points At Intersections  Conflicts occur when traffic streams moving in different directions interfere with each other.  Types of conflicts: Merging, Diverging, and Crossing.  See Figure 8.3.  There are 32 conflict points at four-approach unsignalized intersection.  Number of possible conflict points depend on the number of approaches, the turning movements, and type of traffic control at the intersection.

Conflict Points At Intersections  Primary objective of the design of traffic control system at intersections is to reduce the number of significant conflict points.  In designing a control system it is first necessary to undertake analysis of turning movements that will indicate the significant type of conflicts.  Factors influence significance of a conflict: Type of conflict. Number of vehicles in each conflicting streams. Speeds of vehicles in these streams.

Types Of Intersection Control  Yield Signs.  Stop Signs.  Intersection Channelization.  Traffic Signals.

Yield Signs  All drivers on approaches with yield signs are required to slow down and yield the right of way to all conflicting vehicles at the intersections.  Stopping is not mandatory.  Usually placed on minor roads approaches.  Most significant factor in the warrant of yield signs is the approach speed on the minor road.  Yield sign is warranted at intersections where there is a separate or channelized right-turn lane without an adequate acceleration lane.

Stop Signs  Used where an approaching vehicle is required to stop before entering the intersection.  Should be used only when warranted as they cause considerable inconvenience to motorists.  Should not be used at signalized intersections or on through roadways of expressway.  May be used on a minor road when it intersect a major road at unsignalized intersection.  Also used where combination of high speed, restricted view, and serious crashes indicates the necessity of such control.

Multiway Stop Signs  Require all vehicles approaching the intersection to stop before entering it.  Used as safety measure at some intersections.  Normally used when traffic volumes on all approaches are approximately equal.  When traffic volumes are high ……. Replace by traffic signal.  Warrants: Total intersection volume > 500 veh/h for 8 hours of an average day. Combined volume (vehicles & pedestrians) on minor approach < 200 unit/hr for same 8 hours. Average delay on minor street > 30 sec/veh during the max. hour. The min. requirements for vehicular volume can be reduced by 30% if the 85 th percentile approach speed on the major approach > 40 mi/h.

Intersection Channelization  Used mainly to separate turn lanes from through lanes.  Consist of solid white lines or raised barriers which guide traffic within a lane.  Guidelines for the use of channels at intersections include: Laying out islands or channel lines to allow a natural, convenient flow of traffic. Avoiding confusion by using a few well located islands. Providing adequate radii of curves and widths of lanes for the prevailing type of vehicles.

Traffic Signals  One of the most effectives ways of controlling traffic at an intersection.  Can be used to eliminate many conflicts as traffic streams are assigned to use the intersection at different times.  Cause delay of vehicles therefore should be used only when necessary.  MUTCD describes 11 warrants (most important is traffic volume).  MUTCD warrants should be considered as a guide,  Professional judgment based on experience should be used to decide whether or not to signalize an intersection.

Factors Considered In The warrants For Traffic Signals 1.Eight-hour vehicular volume: 1-IMin. vehicular volume. 1-IIInterruption of continuous traffic. 1-III Combination of warrants. 2.Minimum pedestrian volume. 3.School crossing 4.Coordinated signal system. 5.Crash experience. 6.Roadway network. 7.Four-hour vehicular volume. 8.Peak hour: 8-I Peak hour delay. 8-II Peak hour volume.

Factors Considered In The warrants For Traffic Signals 1-I Minimum Vehicle Volume: Applied when considering traffic volume is the principal factor. When traffic volumes on major streets and on higher- volume minor street approaches for each of any 8 hr of an average day are at least equal to the volumes specified in Table II Interruptions of continuous Traffic: When traffic in minor street suffers excessive delay due to heavy traffic on major street Warrant is satisfied when traffic volume on major street and on the higher-volume minor street approach for each of any 8 hr of an average day is at least equal to the volumes in Table 8.2.

Table 8.1 Volume Requirements For Min. Vehicular Volumes Warrant Veh/h on higher- volume minor street approach (one direction only) Veh/h on major street (total of both approaches) # lane for moving traffic on each approach Minor StreetMajor Street or more or more or more1 Note: If posted, statutory or 85 th percentile speed on the major street > 40 mph or if intersection lies in an area with population < 10,000, then 70% of the volumes shown may be used.

Factors Considered In The warrants For Traffic Signals 1-III Combination of Warrants: When none of the above warrants is satisfied. But when the two warrants are satisfied to the extent of 80% of the volumes in tables 8.1 and Min. Pedestrian Volume: When pedestrian volume crossing major street on an average day is at least 100 for each of any 4 hours or 190 during one hour and there are fewer than 60 gaps per hr that are acceptable by pedestrians for crossing. The nearest traffic signal along the major street should be at least 300 ft away from the intersection. Signal should be of the traffic-actuated type with push buttons for pedestrian crossing.

Table 8.2 Min. Vehicular Volumes For Interruption of Continuous Traffic Warrant Veh/h on higher- volume minor street approach (one direction only) Veh/h on major street (total of both approaches) # lane for moving traffic on each approach Minor StreetMajor Street or more or more or more1 Note: If posted, statutory or 85 th percentile speed on the major street > 40 mph or if intersection lies in an area with population < 10,000, then 70% of the volumes shown may be used.

Factors Considered In The warrants For Traffic Signals 3- School Crossing: When analysis of gap data shows that the frequency of occurrence of gaps and length of gaps are inadequate. Signal should be of the traffic-actuated type with push buttons for pedestrian crossing. All obstruction to view (parked vehicles) should be prohibited for at least 100 ft before and 20 ft after the crosswalk. 4- Coordinated Signal System: When installation will help maintaining a proper grouping of vehicles and effectively regulate group speed. This warrant is not applicable when the resultant spacing of the traffic signal will be less than 300 ft.

Factors Considered In The warrants For Traffic Signals 5- Crash Experience: When crash frequency has not been reduced by adequate trial of less restrictive measures. If 5 or more injury or reported property- damage-only crashes have occurred within 12- month period and that signal control is suitable counter measure for these crashes. In addition, traffic and pedestrian volumes should not be less than 80% of the requirements specified in the min. vehicle volume warrant, the interruption traffic warrant, or the min. pedestrian volume warrant.

Factors Considered In The warrants For Traffic Signals 6- Roadway Network: When installation will help to encourage concentration and organization of traffic networks. 1.If existing or projected volume is at least 1000 during the peak hour of a typical weekday, and the 5-year projected traffic volumes satisfy the requirements of: max. vehicular volume, interruption of traffic, combination of warrants, 4-hr volume, and peak-hour volume. 2.When the total existing or projected entering volume is at least 1000 vehicles for each of any 5-hr of a Saturday or Sunday.

Factors Considered In The warrants For Traffic Signals 7- (4-hour) volume: Based on comparison of standard graphs in the MUTCD (see Figure 8.5). When a plot of any 4-hr volume of an average day falls above standard graph, this warrant is satisfied.

Factors Considered In The warrants For Traffic Signals 8-I Peak-hour delay: When the delay during any four (4) consecutive 15- minutes period on one of the minor street approaches (one direction only) controlled by a stop sign is equal to or greater than specified levels and the same minor street approach (one direction only) volume and the total intersection entering volume are equal to or greater than specified levels. Specified level: 4 vehicle-hours for one lane approach, and 5 vehicles-hours for a two-lane approach. Specified volumes are: 100 veh/hr for one moving lane and 150 veh/h for two moving lanes, and 800 veh/h entering volume for intersections of four or more approaches or 650 veh/hr for intersections with 3 approaches.

Factors Considered In The warrants For Traffic Signals 8-II Peak-hour volume: Based on comparison of standard graphs in the MUTCD (see Figure 8.6). When a plot of an average day falls above standard graph, this warrant is satisfied.