Figure 3. Shared use paths should use materials such as asphalt or colored concrete to visually differentiate the space from a conventional sidewalk. Visibility and safety. Since shared use paths are usually designed to accommodate two-way travel and incorporate a buffer or boulevard space between the path and adjacent roadway, it is important to prioritize user safety and visibility at all conflict points with motorized traffic.
To maintain a consistent and comfortable user experience, shared use paths should attempt to maintain their elevations and cross slopes across intersecting driveways. Where possible, trail pavement materials should be continued across driveways to eliminate the need for horizontal expansion joints and provide additional visual delineation between the path and driveway surface. Shared use paths accommodate pedestrian traffic, and as a result, must maintain ADA-compliance throughout their limits.
The walk interval should provide pedestrians adequate time to perceive the WALK indication and depart the curb before the pedestrian clearance interval begins. It should be long enough to allow a pedestrian that has pushed the pedestrian push button to enter the crosswalk.
In many cases, the pedestrian phase will be set to rest in the walk interval to maximize the walk display during a vehicle green. Some controllers have a mechanism to specify that the walk interval begins before, or even after, the onset of the green interval.
The walk interval may be extended in some controllers during coordination. A pedestrian recall mode, as discussed in a later section, can be used to eliminate the need for a pedestrian to push buttons and ensures that the pedestrian phase is presented each cycle.
The length of the walk interval is usually established in local agency policy. The MUTCD 19indicates that the minimum walk duration should be at least 7 seconds, but indicates that a duration as low as 4 seconds may be used if pedestrian volumes are low or pedestrian behavior does not justify the need for 7 seconds. Consideration should be given to walk durations longer than 7 seconds in school zones and areas with large numbers of elderly pedestrians.
In cases where the pedestrian push button is a considerable distance from the curb, additional WALK time is desirable.
The pedestrian clearance interval follows the walk interval. When the pedestrian clearance interval begins, pedestrians should either complete their crossing if already in the intersection or refrain from entering the intersection until the next pedestrian walk interval is displayed. The MUTCD currently stipulates that the pedestrian clearance interval must be calculated assuming the distance from the curb to the far side of the opposing travel way, or to a median of sufficient width for pedestrians to wait.
Note that previous editions of the MUTCD only required the clearance time to be as long as needed for the pedestrian to reach the center of the farthest traveled lane.
Pedestrian clearance time is computed as the crossing distance divided by the walking speed. The speed of pedestrians is a critical assumption in determining this parameter.
Recent work completed by LaPlante and Kaeser has suggested that a speed of 3. The Pedestrian Facilities User Guide 21 recommends a maximum walking speed of 3. This guide also suggests that a slower walking speed should be used in areas where there is a heavy concentration of elderly persons or children. A survey by Tarnoff and Ordonez 22 suggests a range of 3. Pedestrian clearance time for typical pedestrian crossing distances can be obtained from Table In general, agencies use one of two methods to determine the setting for the pedestrian clearance parameter.
Some agencies require that the pedestrian clearance time conclude with the onset of the yellow change interval. This approach provides additional time equal to the change period for pedestrian clearance—time that is sometimes of benefit to pedestrians who walk slower than average.
The pedestrian clearance interval duration for this practice is computed using Equation Other agencies allow a portion of the pedestrian clearance time to occur during the change period i. This practice minimizes the impact of pedestrian service on phase duration and allows it to be more responsive to vehicular demand. This pedestrian clearance interval duration is computed using Equation The practice of exclusing the change and clearance intervals may place pedestrians at risk if a concurrent permissive left turn movement is receiving a yellow and the vehicles from that movement are expected to clear the intersection during the yellow interval.
Some agencies using flashing yellow applications choose to omit the permissive left turn portion of a protected-permissive left-turn movement during a pedestrian call. Research has shown that the best form of isolated operation occurs when fully-actuated controllers are used.
Actuated controllers operate most effectively when timed in a manner that permits them to respond rapidly to fluctuations in vehicle demand This section describes several of the more commonly used settings and parameters that influence phase function or duration in an actuated controller, including phase recall, passage time, simultaneous gap, and dual entry.
In addition, this section discusses the volume-density technique. Recall causes the controller to place a call for a specified phase each time the controller is servicing a conflicting phase, regardless of the presence of any detector-actuated calls for the phase. There are four types of recalls: minimum recall also known as vehicle recall , maximum recall, pedestrian recall, and soft recall.
The minimum recall parameter causes the controller to place a call for vehicle service on the phase. The phase is timed at least for its minimum green regardless of whether there is demand on the movement.
The call is cleared upon start of green for the affected phase and placed upon start of the yellow change interval. This may be used where detection has failed. Minimum recall is the most frequently used recall mode. It is frequently used for the major-road through-movement phases commonly designated as phases 2 and 6 at semi-actuated non-coordinated intersections.
This use ensures that the controller will always return to the major-road through phases regardless of demand on the major-road through phases, thus providing a green indication as early as possible in the cycle. The maximum recall parameter causes the controller to place a continuous call for vehicle service on the phase. It results in the presentation of the green indication for its maximum duration every cycle as defined by the maximum green parameter for the phase.
This type of operation is typically only used on a time-of-day basis in conjunction with a particular coordinated plan see Chapter 6. Coordination plans may invoke pedestrian calls using a rest in walk command, which dwells in the pedestrian walk interval, while awaiting the yield point. The soft recall parameter causes the controller to place a call for vehicle service on the phase in the absence of a serviceable conflicting call.
When the phase is displaying its green indication, the controller serves the phase only until the minimum green interval times out. The phase can be extended if actuations are received.
This may be used during periods of low traffic when there is a desire to default to the major street. The most typical application for soft recall is for the major-road through movement phases usually phases 2 and 6 at non-coordinated intersections.
The use of soft recall ensures that the major-road through phases will dwell in green when demand for the conflicting phases is absent. Passage time, sometimes called passage gap, vehicle extension, or unit extension, is used to extend the green interval based on the detector status once the phase is green.
This parameter extends the Green Interval for each vehicle actuation up to the Maximum Green. It begins timing when the vehicle actuation is removed. This extension period is subject to termination by the Maximum Green timer or a Force Off. Passage time is used to find a gap in traffic for which to terminate the phase, essentially it is the setting that results in a phase ending prior to its maximum green time during isolated operation.
If the passage time is too short, the green may end prematurely, before the vehicular movement has been adequately served.
If the passage interval is set too long, there will be delays to other movements caused by unnecessary extension of a phase 25, resulting in delay to the other movements at the intersection. The appropriate passage time used for a particular signal phase depends on many considerations, including: type and number of detection zones per lane, location of each detection zone, detection zone length, detection call memory i. Detection design procedures that reflect these considerations are described in Chapter 4.
The passage timer starts to time from the instant the detector actuation is removed. A subsequent actuation will reset the passage timer. Thus, the mode of the detector, pulse or presence, is extremely important in setting the passage time.
The pulse mode essentially measures headways between vehicles and the passage time would be set accordingly. The speed of the vehicles crossing the detectors and the size of the detectors is an important consideration in determining passage time when using presence mode. Longer passage times are often used with shorter detectors, greater distance between the detector and stop line, fewer lanes, and slower speeds. When the passage timer reaches the passage time limit, and a call is waiting for service on a conflicting phase, the phase will terminate, as shown in Figure In the figure, vehicle calls extend the green time until the gap in detector occupancy is greater than the passage time.
In this example, presence detection is assumed. Research by Tarnoff suggests that the vehicle extension interval is one of the most important actuated controller settings, but the variety of techniques for determining proper settings suggest that there is either a lack of knowledge on the availability of this information or disagreement with the conclusions presented The objective when determining the passage time value is to make it large enough to ensure that all vehicles in a moving queue are served but to not make it so large that it extends the green for randomly arriving traffic.
This objective is broadened on high-speed approaches to ensure the passage time is not so large that the phase cannot be safely terminated. Many professionals believe that keeping one lane of traffic in a left turn or a minor street moving in deference to a major street with multiple lanes results in inefficient operation. Research has shown that measuring flow rates across lane groups and comparing them with the potential demand at an approach may provide improved decision making within the signal control logic.
The guidelines provided in this section are based on the assumption that non-locking memory is used and that one source of detection is provided per lane for the subject signal phase. This source of detection could consist of one long detector loop at the stop line, a series of 6-foot loops that are closely spaced and operate together as one long zone of detection near the stop line, or a single 6-foot loop located at a known distance upstream of the stop line and no detection at the stop line.
As discussed in Chapter 4, passage time is a design parameter for detection designs that include multiple detectors for the purpose of providing safe phase termination i.
The passage-time value for this application is inherently linked to the detection design and should not be changed from its design value. Passage time defines the maximum time separation that can occur between vehicle calls without gapping out the phase.
When only one traffic lane is served during the phase, this maximum time separation equals the maximum allowable headway MAH between vehicles. Although the maximum time separation does not equal the maximum allowable headway when several lanes are being served, the term "MAH" is still used and it is understood that the "headway" represents the time interval between calls and not necessarily the time between vehicles in the same lane.
Figure illustrates the relationship between passage time, gap, and maximum allowable headway for a single-lane approach with one detector. This relationship can be used to derive the following equation for computing passage time for presence mode detection. Gap as shown in this figure is the amount of time that the detection zone is unoccupied.
Figure Relationship between passage time, gap, and maximum allowable headway. If Equation is used with pulse-mode detection, then the length of vehicle Lv and the length of detector Ld equal 0. The duration of the passage time setting should be based on three goals 27 :. Research by Tarnoff and Parsonson 28 indicates that there is a range of passage times within efficient intersection operations. This range extends from about 1 to 4 seconds for presence mode detection, with lower values being more appropriate under higher volume conditions.
Values outside this range tend to increase delay. These passage times correspond to MAH values in the range of 2. Based on the previous discussion, the following MAH values are recommended for use with Equation to determine passage time:.
The recommended MAH values may be increased by 0. The passage time computed from the recommended MAH values for a range of speeds and detection zone lengths is provided in Table for presence mode detection. It is critical that the relationship of passage time to vehicle speed, detector length, and detector location be considered. Simultaneous gap defines how a barrier is crossed when a conflicting call is present.
If enabled, it requires all phases that are timing concurrently to simultaneously reach a point of being committed to terminate by gap-out, max-out, or force-off before they can be allowed to jointly terminate. If disabled, each of the concurrent phases can reach a point of being committed to terminate separately and remain in that state while waiting for all concurrent phases to achieve this status.
Simultaneous gap out should be enabled when advance detection is used to provide safe phase termination. The dual double entry parameter is used to call vehicle phases that can time concurrently even if only one of the phases is receiving an active call.
For example, if dual entry is active for Phases 2 and 6 and Phase 1 receives a call but no call is placed on Phase 6, Phase 6 would still be displayed along with Phase 1. The most common use of dual entry is to activate the parameter for compatible through movements.
If the dual entry parameter is not selected, a vehicle call on a phase will only result in the timing of that phase in the absence of a call on a compatible phase. Volume-density features can be categorized by two main features: gap reduction and variable initial.
These features permit the user to provide variable alternatives to the otherwise fixed parameters of passage time gap reduction and minimum green variable initial.
Gap reduction provides a way to reduce the allowable gap over time, essentially becoming more aggressive in looking for an opportunity to end the phase. Variable initial provides an opportunity to utilize cycle by cycle traffic demand to vary the minimum time provided for a phase. These features increase the efficiency of the cycle with the fluctuations in demand, which can result in lower delay for users at the intersection.
The gap reduction feature reduces the passage time to a smaller value while the phase is green. This left-turn phase sequence is most commonly used in coordinated systems with closely spaced signals, such as diamond interchanges. It has both opposing left-turn phases ending at the same time. If it is implemented in a single ring structure, then the two phases also start at the same time. If a dual-ring structure is used, then each left-turn phase is assigned to a different ring such that each can start when the left-turn demand is served i.
Lagging left-turn phasing is also recognized to offer operational benefits for the following special situations:. When used with protected phasing, this phase sequence provides a similar operational efficiency as a lead-lead or lead-lag phase sequences. However, differences emerge when they are used with protected-permissive mode.
One disadvantage of lagging left-turn phases is that drivers tend not to react as quickly to the green arrow indication. Another disadvantage is that, if a left-turn bay does not exist or is relatively short, then queued left-turn vehicles may block the inside through lane during the initial through movement phase.
When lag-lag phasing is used at a four-leg intersection where both phases are used with the protected-permissive mode, then both left-turn phases must start at the same time to avoid the "yellow trap" or left-turn trap problem, illustrated in Figure This problem stems from the potential conflict between left-turning vehicles and oncoming vehicles at the end of the adjacent through phase.
Of the two through movement phases serving the subject street, the trap is associated with the first through movement phase to terminate and occurs during this phase's change period. The left-turn driver seeking a gap in oncoming traffic during the through phase, first sees the yellow ball indication; then incorrectly assumes that the oncoming traffic also sees a yellow indication; and then turns across the oncoming traffic stream without regard to the availability of a safe gap.
In fact, under at least one condition, the second technique can operate more efficiently than dual-ring lead-lead phasing. This condition occurs when the left-turn volume is moderate to heavy and relatively equal on both approaches. Regardless, a detailed operational evaluation should always be used to confirm that lag-lag phasing operates more efficiently than other phasing options. The third technique avoids the yellow trap by using an overlap in the controller and a five-section left-turn signal head.
An overlap is a controller output to the signal head load switch that is associated with two or more phases. In this application, the left-turn green and yellow arrow indications are associated with the subject left-turn phase; and the left-turn green, yellow, and red ball indications are associated with the opposing through movement phase as opposed to those of the adjacent through phase. The flashing yellow arrow is contained within a three-, four-, or five-section head and provides a permissive indication to the driver that operates concurrent with the opposing through movement rather than the adjacent through movement.
This study was conducted over a 7-year period and comprised a very comprehensive research process, including engineering analyses, static and video-based driver comprehension studies, field implementation, video conflict studies, and crash analyses. This study 11 recommended that a flashing yellow arrow be allowed as an alternative to the circular green for permissive left-turn intervals.
The louvered signal head is referred to as the "Dallas Display. This left-turn phase sequence is generally used to accommodate through movement progression in a coordinated signal system.
The aforementioned "yellow trap" may occur if the leading left-turn movement operates in the protected-permissive mode and the two through movement phases time concurrently during a portion of the cycle. The "yellow trap" problem can be alleviated by using one of the following techniques:.
The first two techniques will likely have an adverse effect on operations, relative to a dual ring implementation of lead-lag phasing with protected-permissive operation. However, they avoid the potential adverse effect a yellow trap would have on safety. However, in practice, the Dallas Display is used for both the leading and the lagging left-turn signal heads because it improves operational performance Lead-lag phasing is also recognized to offer operational benefits for the following special situations:.
Pedestrian movements are typically served concurrently with the adjacent through movement phase at an intersection. This is done to simplify the operation of the intersection primarily and is largely a legacy issue in our application of signal logic and control.
Typical application of pedestrian operation puts pedestrians in conflict with right-turning vehicles and left-turning vehicles that operate in a permissive mode, by inviting their movement at the same time. There are specific measures that can be used to mitigate this potential conflict, three common options include:.
Figure Ring-barrier diagrams showing a leading pedestrian interval and an exclusive pedestrian phase. Two types of right-turn phasing are addressed in this section. The first type is based on the addition of a phase to the signal cycle that exclusively serves one or more right-turn movements. This type of right-turn phasing is rarely used. If it is being considered, then its operational or safety benefits should be evaluated and shown to outweigh its adverse impact on the efficiency of the other intersection movements.
The second type of right-turn phasing is based on the assignment of the right-turn movement to the phase serving the complementary left-turn movement on the crossroad. The following conditions should be satisfied before using this type of right-turn phasing:.
If the aforementioned conditions are satisfied, then the appropriate operational mode can be determined. If the through movement phase for the subject intersection approach serves a pedestrian movement, then the right-turn phasing should operate in the protected-permissive mode.
As shown in Figure , the permissive right-turn operation would occur during the adjacent through movement phase, and the protected right-turn operation would occur during the complementary left-turn phase. If the through movement phase for the subject intersection approach does not serve a pedestrian movement, then the right-turn phasing should operate in the protected only mode during both the adjacent through movement phase and the complementary left-turn phase.
A controller overlap may be used to provide this sequence. Detectors place calls into the traffic signal controller. The controller uses this information and the signal timing to determine the display provided to the users.
Detection for pedestrians is limited in most cases to push buttons as shown in Figure , although accessible 20 pedestrian signal detectors are increasing in their use. There are various forms of vehicle detection technologies, and strengths and weaknesses of each are described in the Traffic Detector Handbook, 3rd edition The detection design for an intersection describes the size, number, location, and functionality of each detector.
Most engineering drawings include the wiring diagram for how detectors are associated to phases. Signal timing settings such as the passage time, delay, extend, and other related parameters are described in more detail in Chapter 5.
The size and location of detectors is an important element in traffic signal design. Detectors can consist of one 6-foot-byfoot inductive loop detector, a series of closely spaced 6-foot-byfoot loop detectors may be circular in shape as shown in Figure , one long 6-foot-byfoot loop detector, or alternative detection technology e. This detection zone can be used to meet the objectives described below.
A call can be triggered by an actuation from any detection, vehicular, pedestrian, or other or through a controller function. These parameters are described in Chapter 5. The objective of detection is to detect vehicle presence and identify gaps in vehicle presence that are sufficiently long to warrant terminating the phase. There are many objectives of detection design that can be characterized with the following statements:. The first and fourth objectives are safety related.
The first objective addresses expectancy, while the fourth specifically addresses the potential crashes as a result of phase termination. The fourth objective is achieved by using advance detectors on the approach. The location of these detectors can vary and depends on the detection technology used as well as intersection approach speed. The safety benefit of this design tends to be more significant on high-speed approaches.
The other objectives focus on intersection efficiency. The second and third objectives are designed to address efficiency. During low volume late night or off-peak conditions, detection should seek to serve all traffic identified without stopping.
In peak conditions, green allocation should seek to measure flows and maintain flows that are near saturation flow as described in Chapter 3. Detection timing to achieve this objective will be discussed in more detail in Chapter 5. The detection operating mode refers to the way the detection unit measures activity and is set in the detection unit. It also affects the duration of the actuation submitted to the controller by the detection unit.
One of two modes can be used: presence or pulse. The presence mode is typically the default mode. Pulse mode is used to describe a detector which detects the passage of a vehicle by motion only point detection. The actuation starts with the arrival of the vehicle to the detection zone and ends after the pulse duration.
This mode is typically used when the detectors are located upstream of the stop line and the associated detector channel operates in the locking mode. Presence mode is used to measure occupancy and the actuation starts with the arrival of the vehicle to the detection zone and ends when the vehicle leaves the detection zone.
Thus, the time duration of the actuation depends on the vehicle length, detection zone length, and vehicle speed. Presence mode measures the time that a vehicle is within the detection zone and will require shorter extension or gap timing with its use.
Presence mode is typically used with long-loop detection located at the stop line. In this application, the associated detector channel in the controller is set to operate in the non-locking mode see next section. In this mode, the delay or extend parameters in the controller described in detail in Chapter 5 can be used to modify the call start and end times. Alternatively, the delay or extend functions in the detection unit could also be used to adjust the start and end time of the actuation.
The combination of presence mode operation and long-loop detection typically require a small passage time value to maintain efficiency. This characteristic tends to result in an operational benefit through efficient queue service. Modifiers to the detector settings are commonly handled in the controller. This has increased in popularity because it provides a single location for information on all phases at the intersection.
One of two modes can be used: non-locking or locking. This mode is set in the controller for each of its detector channel inputs. It dictates whether an actuation received during the red interval and optionally, the yellow interval is retained until the assigned phase is served by the controller.
All actuations received during the green interval are treated as non-locking by the controller. The non-locking mode is typically the default mode. In the non-locking mode, an actuation received from a detector is not retained by the controller after the actuation is dropped by the detection unit. The controller recognizes the actuation only during the time that it is held present by the detection unit. In this manner, the actuation indicates to the controller that a vehicle is present in the detection zone and the controller converts this actuation into a call for service.
This mode is typically used for phases that are served by stop line detection. It allows permissive movements such as right-turn-on-red to be completed without invoking a phase change. In doing so, it improves efficiency by minimizing the cycle time needed to serve minor movement phases.
Non-locking mode is not typically used with pulse detection due to an inability to detect vehicle presence after the pulse duration elapses. In the locking mode, the first actuation received by the controller on a specified channel during the red interval is used by the controller to trigger a continuous call for service. This call is retained until the assigned phase is serviced, regardless of whether any vehicles are waiting to be served.
This mode is typically used for the major-road through movement phases associated with a low percentage of turning vehicles as may be found in rural areas. One advantage of using this mode is that it can eliminate the need for stop line detection, provided that advance detection is provided and that it is designed to ensure efficient queue service. Detection designs for high speed approaches speeds greater than 35 mph have the objective to not only service the queue at the beginning of green but also to safely terminate the phase in the presence of a conflicting call.
Stop bar detection is usually used to clear the queues and the multiple upstream detectors are used to safely terminate the phase. For efficient operation, the stop bar detector should be programmed as a queue detector so that the stop bar detector is disconnected after the queue clears and only the upstream detectors are used to safely terminate the phase.
When stop bar detectors are not used, volume density functions should be used to provide appropriate minimum green time to clear the queues. The design of advance detection on high-speed approaches requires special attention. Drivers within a few seconds travel time of the intersection tend to be indecisive about their ability to stop at the onset of the yellow indication. The location of this zone is shown in Figure The zone boundaries obtained by these three definitions are compared in Figure The boundaries based on distance typically have an exponential relationship.
Those based on travel time have a linear relationship. Based on the trends shown in the figure, the beginning and end of the indecision zone tend to be about 5. These times equate to about the 90th-percentile and 10th-percentile drivers, respectively. For these types of designs, the furthest detector upstream of the stop bar is usually located at the beginning of the indecision zone of the approach design speed 85th-percentile approach speed.
This is usually at a distance of 5 to 5. Subsequent detectors have a design speed of 10 mph lower than the upstream detector. Typically 3 to 4 detectors are used to enable safe termination of the high speed approach phase.
The detectors are allowed to extend the phase by the passage time programmed in the controller or by the extension time on the detector itself see Chapter 5. Although the concept of the indecision zone has been known for many years, comprehensive research has not been completed to conclude what the appropriate way to address the human factors associated with intersection design and signal timing display. The indecision should not be confused with the dilemma faced by drivers determining whether there is distance to stop and, if not, to travel through the intersection before a conflicting movement receives a green indication.
Detection design for low speed traffic movements speeds of 35 mph or less has a different objective compared to the detection design for high speed traffic movements. The primary objective of detection on low speed approaches is to call a phase and clear the queue while minimizing delay. Due to lower speeds, there is less emphasis on protection from dilemma zone or indecision zone on the approach. Hence, some agencies use only stop bar detection for low speed approaches.
Stop bar detectors are usually operating in the presence mode. This facilitates the primary objective of detector calling the phase and clearing the queues. The key element of this design is the determination of detection zone length. The optimal length represents a trade off in the desire to avoid both premature gap out and excessive extension of green. According to Lin 20 , the ideal length of the stop line detection zone is about 80 feet. Description : This three-day Traffic Signal Design course is designed to enable the participants to obtain an understanding of the fundamental concepts and MnDOT standard practices related to the design of traffic signal systems within the State of Minnesota.
This class will also provide design training for the flashing yellow left turn arrow. Schedule : Offered once every year. Last offered: March 25, Next Class : Spring Audience : Anyone interested in traffic signals including state, county, city and consultant personnel involved in traffic signals. Description : This is a one-day introductory traffic signal course designed to enable entry-level traffic personnel to acquire a basic knowledge of a traffic signal.
0コメント