National Academies Press: OpenBook

Design, Operation, and Safety of At-Grade Crossings of Exclusive Busways (2007)

Chapter: Chapter 6 - Operational Practices

« Previous: Chapter 5 - Traffic Control Devices
Page 21
Suggested Citation:"Chapter 6 - Operational Practices." National Academies of Sciences, Engineering, and Medicine. 2007. Design, Operation, and Safety of At-Grade Crossings of Exclusive Busways. Washington, DC: The National Academies Press. doi: 10.17226/23171.
×
Page 21
Page 22
Suggested Citation:"Chapter 6 - Operational Practices." National Academies of Sciences, Engineering, and Medicine. 2007. Design, Operation, and Safety of At-Grade Crossings of Exclusive Busways. Washington, DC: The National Academies Press. doi: 10.17226/23171.
×
Page 22
Page 23
Suggested Citation:"Chapter 6 - Operational Practices." National Academies of Sciences, Engineering, and Medicine. 2007. Design, Operation, and Safety of At-Grade Crossings of Exclusive Busways. Washington, DC: The National Academies Press. doi: 10.17226/23171.
×
Page 23
Page 24
Suggested Citation:"Chapter 6 - Operational Practices." National Academies of Sciences, Engineering, and Medicine. 2007. Design, Operation, and Safety of At-Grade Crossings of Exclusive Busways. Washington, DC: The National Academies Press. doi: 10.17226/23171.
×
Page 24
Page 25
Suggested Citation:"Chapter 6 - Operational Practices." National Academies of Sciences, Engineering, and Medicine. 2007. Design, Operation, and Safety of At-Grade Crossings of Exclusive Busways. Washington, DC: The National Academies Press. doi: 10.17226/23171.
×
Page 25

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

21 This chapter presents general considerations for operating signalized, at-grade crossings of exclusive busways. It is closely related to the chapter on traffic control devices. Signal Placement and Positioning The MUTCD, Section 4D, provides guidance for the place- ment and positioning of traffic signals at traditional intersec- tions (10). Additional guidance is provided in Chapter 13 of the Traffic Engineering Handbook (2) and in the Traffic Con- trol Devices Handbook (12) and is not repeated here. Instead, this section presents additional considerations for busway intersections given their unique design and particular safety issues. As a general rule, the placement of traffic signal dis- plays should be consistent from intersection to intersection, to improve driver and pedestrian expectancy. Median Busway Intersections The primary signal placement concern at busway intersec- tions is the placement and positioning of the busway signal in relation to the same-direction, parallel-street traffic signals so that motorists do not confuse the busway signals with their own. This concern is particularly important for left-turning motorists. Separating the bus signal horizontally or vertically (where practical) from the left-turn signal can help to avoid confusion with the signals. Chapter 5, Traffic Control Devices, identified some options for this including the use of white bar signal indications. White bar signals provide a dis- tinct message that motorists are not trained to understand. Other possibilities include programmable visibility lenses, louvers, or visors. Of these methods, white bar signals are the most desirable. The MUTCD recommends using signal visors in lieu of signal louvers. As a result, signal louvers are not dis- cussed here. Programmable visibility lenses, also called visibility- limited signals, limit the field of view of a signal (13). They allow greater definition and accuracy of the field of view than louvers. They are particularly well suited for lateral (horizon- tal) separation. In this case, the lateral separation is between the busway signal and the left-turn lane. Programmable lenses must be clearly targeted at the intended lane and should only be used with rigid mountings such as a mast arm or pole. Regular in-field maintenance should be conducted to ensure the alignment of the signal. Some agencies using visibility-limited signals at busway intersections have received complaints from motorists that note the signals in the paral- lel lane are malfunctioning. Visors are used to improve the visibility of signals in direct sunlight by providing additional contrast between the signal lens and the background. The MUTCD requires their use when the angle between intersection roadways is relatively small. However, the signal indications can likely still be viewed by parallel directions of travel as in the case of median busway intersections. As part of the case study visits, agencies were asked if posi- tioning the busway signals in another location would help to separate their view from motorists. All studied agencies pre- ferred to have the busway signals directly over the center of the busway lane. Side-Aligned Busway Intersections As with median busway intersections, limiting the view of busway signals from the same-direction, parallel traffic is a safety concern at side-aligned busway intersections and the same countermeasures are appropriate. At side-aligned busway intersections, the primary concern is right-turning vehicles from the parallel street. Another concern at side-aligned busway intersections is the spacing of the intersection of the cross street and the busway and the cross street and the parallel street. Depending on the spacing, two sets of signals, operating from one con- troller, are likely needed for the cross street. Cross-street C H A P T E R 6 Operational Practices

motorists may be looking downstream at the parallel roadway signal and may not see the busway signal. The distance between these two signals is the longitudinal separation or the distance separation. Programmable signals are useful in lim- iting the longitudinal visibility and can be used in this sce- nario. As with separated right-of-way intersections, larger signal heads and back plates can increase the visibility of these intersections. Separated Right-of-Way Intersections As previously discussed, the primary safety concern at sep- arated right-of-way busway intersections are signal violations by vehicles on the cross-street approach. Therefore, traffic sig- nals for the cross-street traffic should be designed for greatest visibility and conspicuity. Signal heads placed in accordance with the MUTCD should be visible to all motorists approaching the intersec- tion. Although the MUTCD requires a minimum of two sig- nal faces be provided for the major movement on an approach, placing one signal head over each lane for multi- lane roadways will improve their visibility. When a signal head is positioned over the middle of a lane, it is in the cen- ter of the motorist’s cone of vision, thereby increasing its vis- ibility. The additional signal head further increases the likelihood that a motorist will see the signal display for the approach. The MUTCD does not require that signals be placed over- head rather than mounted on poles. However, overhead- signal displays generally provide better conspicuity and are in the motorists’ direct line of sight. The view of pole-mounted signals is more likely to be occluded by another vehicle approaching the intersection. Pole-mounted signals may be useful as supplements to the overhead signals, particularly if there are concerns about sight distance. Another method to increase the visibility of traffic signals is to install 12-inch signal lenses instead of 8-inch signal lenses, a 125% increase in the area of the signal face. Measures also may be needed to prevent sun glare. Conspicuity of the traffic signal is another consideration. Two methods that are applicable at separated right-of-way busway intersections are back plates on the signal heads and the use of LED lenses. Back plates improve the conspicuity of the signal by providing a black background around the signals, thereby enhancing the contrast. LED units are brighter than incandescent bulbs. They also are very energy efficient and have a longer life, increasing the replacement interval (13). Left- and Right-Turn Treatments Treatments for turning motor vehicles are a concern at median busway intersections and at side-aligned busway intersections. At separated right-of-way intersections, no turns are made by general traffic; therefore, this section does not address separated right-of-way intersections. Left Turns from the Parallel Street Left turns by motorists from the parallel roadway, across busway intersections, should always be protected, without exception. The optimal phasing for the protected left turn whether before the parallel through-vehicle phase (leading) or after (lagging) is disputable. The concern for a leading left-turn phase is that vehicles may attempt to make a left turn at the end of the protected phase, in essence trying to “beat the signal,”and turn into the path of an oncoming bus. However, a concern for a lagging phase is that left-turning motorists may move in response to the moving of parallel through traffic. The most recent study of the effect of leading versus lagging left-turn phasing on crashes, although not in relation to busways, found that phasing should be based on considerations other than left- turn head-on crash potential (14). A TCRP study on the inte- gration of light rail signals into city streets found that motorists violated red left-turn arrow indications in left-turn lanes paral- lel to light rail when the leading left-turn signal phase was pre- empted by an approaching light rail vehicle (15). Although the median busway intersections along the 98 B-Line in Richmond, British Columbia, have leading left-turn phasing, violations of red left-turn arrow indications have not been found to be a problem; however, the 98 B-Line does not preempt signal phases. Similarly, no problems have been found to date on the Orange Line in Los Angeles, which uses leading left-turn phas- ing at the median busway intersections. Left Turns from the Cross Street Left turns from the cross street can be protected, permitted, or protected-permitted depending on the characteristics of the intersection and the signal phasing employed. If the cross street has high opposing volumes and the available gaps for a left turn are limited, protected-only phasing should be used to avoid left-turn queues backing up over the busway at the end of the cross-street phase. Another option is to use split phas- ing for the cross-street movements. Left-Turn Prohibition Sometimes, it may be necessary to prohibit left turns at median busway intersections or left turns from the parallel street of side-aligned busway intersections. Reasons for pro- hibiting left-turning movements include • Median intersection design does not have adequate room to accommodate turning radius; • Intersection right-of-way is not sufficient to provide a dedicated left-turn lane; 22

• The signal cycle does not have sufficient time to allow a left- turn phase and accommodate all of the traffic demands at the intersection; and • Left-turning vehicles queue over the busway because of downstream traffic congestion. If left turns are prohibited, a green through-arrow signal should be used for the through signal indications, particularly for the signal on the left lane. The intersections also should use appropriate turn-prohibition signs as discussed in Chap- ter 5 and should prohibit U-turns. If left turns at the intersection are a concern for either safety or operations, other options are available for providing the left turns away from the intersection. Unconventional intersection designs such as jughandles and median U-turns can be used. Figure 6-1 illustrates the vehicle movements at a jughandle intersection. One-way street networks for cross-street traffic also will reduce the turn movements at intersections and thereby reduce the number of conflicts at each intersection. If left turns are prohibited altogether at the intersection, both left turns and U-turns may need to be allowed at down- stream intersections to compensate for this prohibition. In essence, prohibiting the left turns at one intersection may shift the problem to another intersection. Generally, left turns should not be prohibited entirely along a busway corridor. Motorists will seek out other methods to accomplish their turns that may include violating the prohi- bitions, which will cause a severe safety problem. Right-Turn Prohibition At some side-aligned busway intersections, right turns from the parallel street may need to be prohibited, particu- larly if the right-turn-on-red prohibition is often violated or if there is not sufficient room for a right-turn-only lane. As with the left-turn prohibition, a green through arrow should be used for the through signal indications, particularly 23 Source: Signalized Intersections (3). Figure 6-1. Vehicle movements at a jughandle intersection. Figure 6-2. Two-phase signal phasing for separated right- of-way busway intersections. Figure 6-3. Example signal timing for median busway intersection. for the signal head on the right side of the approach. The appropriate turn-prohibition signs should also be used as dis- cussed in Chapter 5. Signal Operation Phasing Example signal phasing plans are provided in this section. These phases are only intended to be examples as the unique geometry, traffic volumes, bus volumes, and pedestrian and bicycle volumes should be considered for each intersection. An example of a basic signal phasing for a separated right- of-way busway intersection is displayed in Figure 6-2. A very simple signal phasing is used. A separated right-of-way inter- section generally only has two phases: the cross-street phase and the busway phase. In most cases, no turns are allowed at the intersection so additional phases are not needed. If the intersection is used as an exit point from the busway, the bus operators could make permissive turns during the busway phase without adding any phases. If the intersection is used as an entry point to the busway, an additional bus-only turning phase would be added. An example of a signal phasing for a median busway inter- section is displayed in Figure 6-3. The left turns from the mainline follow the busway and through-vehicle phase. These are lagging left turns. This example assumes that there is ade- quate pedestrian storage and facilities in the median to accommodate pedestrians crossing to the median during the left-turn phase. A variation on this example is to operate cross traffic in separate phases (i.e., split phases).

An example of a signal phasing for a side-aligned busway intersection is displayed in Figure 6-4. In this example, vehi- cles are not allowed to store between the intersection of the cross street with the busway and the intersection of the cross street with the parallel roadway. Cross-street traffic operates in separate phases. Although not pictured, significant clear- ance intervals are needed after each of the side street phases to ensure that vehicles completely clear the busway intersec- tion. The crossing opportunities for pedestrians are limited across the mainline. Cycle Length The cycle length depends on the traffic volumes at the intersection and the block spacing where the intersection is part of a coordinated system. The more phases in the signal, the longer the cycle length. Longer cycle lengths should gen- erally be avoided to reduce overall intersection delay. Generally, cycle lengths should not exceed 90 seconds at smaller intersections and 120 seconds at larger, more complex intersections. Longer cycle lengths increase the delay of inter- section users. Pedestrians are particularly sensitive to delay and may violate the pedestrian signal if the delay is too long. At median busway intersections, the time needed for pedestrians to cross the general-purpose lanes and the busway will likely be the limiting factor. The cycle length should be as short as possible while giving pedestrians enough time to cross the intersection safely. Yellow and All-Red Intervals As defined by the MUTCD, the yellow change interval is the first interval following the green interval during which the yel- low signal indication is displayed. The exclusive function of the yellow change interval is to warn traffic of an impending change in the right-of-way assignment. The yellow interval should be of sufficient length to allow time for motorists to see the yellow signal indication and decide whether to stop or enter the intersection. It should allow motorists farther away from the signal to decelerate comfortably in advance of enter- ing the intersection and motorists closer to the signal to enter the intersection during the yellow indication. Some agencies also use an all-red clearance interval after the yellow interval during which all signal indications display the red indication. The yellow and all-red intervals are collectively known as the change period. There is currently no nationally recog- nized recommended practice for determining the change period length. The following kinematic equation is used by many agencies to calculate the change period, CP: [6-1] Where t = motorist perception-reaction time,generally 1 second; V = speed of the approaching vehicle, in ft/s; a = deceleration rate, typically 10 ft/s2; g = grade of approach, in percentage divided by 100 (downhill is negative); W = width of the intersection, in feet; and L = length of vehicle, typically 20 feet. One well-recognized practice for using this equation is to allocate the first two terms to the yellow interval and the third term to the all-red interval. This practice will work well for bus- way intersections. However, two important changes should be made to the inputs. First, when the change period is calculated for busway movements, the length of the vehicle should be changed to the length of the buses operating on the busway. CP t V a g W L V = + + + + 2 64 4. 24 Figure 6-4. Example phasing for a side-aligned busway intersection with no storage.

While most transit buses are 40 feet long, many busways use articulated buses that are up to 60 feet long. Second, median and side-aligned busway intersections are much wider than traditional intersections. The longest travel path for each phase should be considered as the intersection width. The travel path should be calculated from the stop bar of the movement all the way to the far side of the crosswalk on the receiving approach (16). In most cases, this calculation will result in long change periods. The MUTCD provides guidance that the red clear- ance interval should have a duration not exceeding 6 seconds. However, this is guidance, not a standard. Signal Coordination System Progression A well-timed corridor signal system benefits intersection operations and safety. It reduces overall intersection delay, reduces stops, and decreases emissions. A progressed corridor may also reduce traffic signal violations. Progression of roadways that cross busway intersections may also benefit the safety of the busway. At side-aligned and separated right-of-way busway intersections, motorists may fail to see the busway intersection and violate the signal. A well-progressed system could eliminate the need for cross- street traffic to stop at these intersections. Busway Priority and Preemption The MUTCD defines preemption control as the transfer of normal operation of a traffic control signal to a special con- trol mode of operation. Priority control is a means by which the assignment of right-of-way is obtained or modified. The systems visited as part of the case studies used a variety of busway priority systems to reduce travel delays on the busway, but no preemption systems. Transit signal priority (TSP) can include extending the green interval for the busway, providing an early green phase for the busway by shortening the green interval of another movement, providing the busway phase before the phase of another move- ment, or inserting a special phase to assist the bus in entering the travel stream ahead of the platoon of traffic. There are two types of TSP systems: unconditional and conditional. Unconditional TSP provides the bus with pri- ority every time the bus approaches the intersection. This system is less expensive because it can be implemented with infrastructure-based detection such as loop actuation. Depending on the type, sensitivity, and offset distance of the detection system used, detection systems that are only infrastructure-based could cause unnecessary actuation by buses traveling in the opposite direction. Combining the infrastructure detection with a transponder on the vehicle can reduce the number of false actuations. An example of an unconditional TSP system is the Euclid Corridor median busway in Cleveland, Ohio. It will have an unconditional TSP system including green extensions, early green, and the ability to jump phases. The green extension will be 10 seconds long. The detection is coordinated with the BRT bus door closing at stations. The TSP will not be used at a few intersections along the corridor where the cross streets are part of a coordinated system. Initially, the Euclid Corridor Transportation Project team considered using loop detection at the intersections. How- ever, loop detection involves considerable maintenance and is difficult to reprogram. Instead, video detection by a variable focal length camera will be used. The programmable detec- tion zone will be set at each of the intersections and can be modified if needed. The video detection will sense the pres- ence of the bus and then communicate directly to the signal system. There is no need for communication between the bus and the signal system. However, the video system will not dif- ferentiate between an unauthorized vehicle and a bus. Conditional TSP systems can operate with a schedule- adherence system or a headway-based system. Priority is only provided to the bus if it is behind schedule by some predeter- mined amount of time (e.g., 4 minutes) or if the headway between buses is longer than desired. Conditional priority can be very difficult to employ because the system must have a method of communication among the bus, a central pro- cessing center, and the signal. The advantage is that it reduces the unnecessary demands on the cycle length. It can be con- trolled so that there are a maximum number of cycles where priority is provided. This type of priority is well suited for congested intersections where intersection delay is an impor- tant concern. Surrounding Road Networks Congested downstream intersections on the mainline and the cross street could cause queuing over the busway intersec- tions. Steps should be taken to avoid such congestion as it impacts both the safety and operation of the intersection. Steps may involve changing the signal timings at the down- stream intersection or modifying the signal timings at the busway intersection in response to these queues. For example, far-side loop actuation can be used to sense the presence of a queue in a receiving lane. The traffic movement that is received by that lane can be held until the queue clears. Traffic control devices and enforcement also can be used to deter motorists from entering the intersection under these circumstances. 25

Next: Chapter 7 - Busway Intersection Design »
Design, Operation, and Safety of At-Grade Crossings of Exclusive Busways Get This Book
×
 Design, Operation, and Safety of At-Grade Crossings of Exclusive Busways
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s Transit Cooperative Research Program (TCRP) Report 117: Design, Operation, and Safety of At-Grade Crossings of Exclusive Busways explores planning, designing, and operating various kinds of busways through roadway intersections. The report examines at-grade intersections along busways within arterial street medians; physically separated, side-aligned busways; busways on separate rights-of-way; and bus-only ramps. The intersections highlighted include highway intersections, midblock pedestrian crossings, and bicycle crossings. Appendixes A through I of the contractor’s final report were published as TCRP Web-Only Document 36.

READ FREE ONLINE

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!