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

Chapter: Chapter 4 - Intersection Geometry Controls and Guidelines

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Suggested Citation:"Chapter 4 - Intersection Geometry Controls and Guidelines." 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.
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Suggested Citation:"Chapter 4 - Intersection Geometry Controls and Guidelines." 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 10
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Suggested Citation:"Chapter 4 - Intersection Geometry Controls and Guidelines." 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 11
Page 12
Suggested Citation:"Chapter 4 - Intersection Geometry Controls and Guidelines." 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 12
Page 13
Suggested Citation:"Chapter 4 - Intersection Geometry Controls and Guidelines." 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 13

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9Intersection design geometry should permit the safe and efficient movement of cars, buses, trucks, pedestrians, and bicyclists. It should consider the characteristics of all users, the surrounding environment, and appropriate public agency policies and resources. The resulting design process leads to a coordination of intersection geometry and traffic control devices. It results in appropriate sight distance, lane widths and clearances, and length of turning radius and islands. This chapter describes these elements of design and is primarily based on A Policy on Geometric Design of High- ways and Streets (1). Intersection design should consider both the physical and functional areas of an intersection. The physical area includes the actual intersection while the functional area of an inter- section also includes perception reaction distance, maneuver distances, and queue storage distances. Human and Driver Factors Human and driver factors such as perception reaction time, eye height, and pedestrian walking speeds are important design controls for intersections. These factors are shown in Table 4-1. Vehicle Characteristics Vehicle characteristics such as length, width, height, wheel base, and acceleration/deceleration influence key intersection design elements including lane width, turning radius, storage requirements, and safe stopping sight distance. These basic characteristics are presented in Table 4-2. The dimensions and turning radius for vehicles commonly found at urban and suburban intersections are set forth in Table 4-3. These values were extracted from AASHTO’s A Pol- icy on Geometric Design (1). Please note that this table does not represent all vehicles. Detailed design characteristics of 40-foot, 45-foot, and 60-foot (articulated) buses compiled from various sources are shown in Table 4-4. These buses do not represent all possible buses and characteristics but instead represent a range of common buses. These dimensions translate into the following design crite- ria for trucks and buses. Height The maximum vehicle height is about 12 to 13 feet for urban buses and 13.5 feet for trucks. This height translates into a minimum of 14- to 16-foot vertical clearances, respectively, when allowance is made for pavement resurfacing. Width The maximum vehicle width is 8.5 feet. When outside mir- rors are added on both sides, vehicle envelopes become larger. Bus envelopes, for example, typically become 10 to 10.5 feet. Therefore, 11 feet is suggested as the minimum lane width for buses and tractor-trailer trucks. In cities where buses have greater outside mirror-to-mirror dimensions, wider lanes may be desirable. Length The minimum station (stop) length should be at least 50 feet for 40- to 45-foot buses and 65 feet for 60-foot articulated buses. If more than one bus is expected to dwell at the station, longer stations are necessary. Designs should provide for at least two loading positions, resulting in station lengths of 100 feet (for 40- to 45-foot buses) and 140 to 150 feet (for 60-foot buses). Turning Radius The AASHTO data suggest a minimum outside turning design radius of 45 feet. However, as shown in Table 4-4, C H A P T E R 4 Intersection Geometry Controls and Guidelines

modern buses (with overhang) require up to 51 feet outside turning radius. Bicycle racks on buses would add about 1.5 feet. These data suggest a minimum outside turning radius of at least 55 feet if buses will be turning at intersections. Actual horizontal curve design for buses (and trucks) should consider a simple curve with tapered or three centered compound curves wherever possible. Design Designations “Design designations” form the basic controls for which an intersection is designed. They normally cover elements such as degree of access control, design drivers and vehicles, design years, design daily and peak-hour volumes, and design speeds. Design designations for intersections reflect those for the roadways along which the busway intersections are located. Design Driver Bus operators on busways are professional drivers who are trained to be more aware of potential conflicts than other motorists. However, at busway intersections, motorists on the crossing streets, although licensed, have no professional train- ing and may be unfamiliar with the surrounding environ- ment. These motorists should be considered as the controlling intersection design drivers. Design Vehicle The design vehicle is the largest vehicle expected at the intersection, with reasonable frequency, during the design year. At intersections of state highways and city streets that serve buses with relatively few large trucks, a city transit bus or intercity bus may be used as the design vehicle, depending on local circumstances. Turning radii of design vehicles are important in design- ing corner radii, channelizing islands, and turning roadways. 10 Factor Value Perception/Reaction Time 1.0 - 4.0 secs Driver Height of Eye 3.5 ft Pedestrian Walking Speeds 3.0 - 4.0 ft/sec Source: Adapted from A Policy on Geometric Design of Highways and Streets (1) and Traffic Engineering Handbook (2). Table 4-1. Driver and human factors. Vehicle Characteristics Intersection Design Elements Affected Length Length of storage lanes Length of bus stops and stations Width Width of lanes Width of turning roadways Height Placement of overhead traffic signals and signs Vertical clearance under overcrossings Wheelbase/Overhang Island nose placement Corner radius Width of turning roadways Acceleration Rates Acceleration tapers and lane lengths Deceleration Rates and Braking Capabilities Deceleration tapers and lane lengths Safe stopping sight distance Source: Adapted from NCHRP Report 279 (6). Table 4-2. Effect of vehicle characteristics on intersection design. Minimum Turning Radius Type Symbol Height (ft) Width (ft) Length (ft) Inside (ft) Design (ft) Passenger Car P 4.25 7.0 19.0 14.4 24.0 Single Unit Truck SU 11-13.5 8.0 30.0 28.3 42.0 Intercity Bus BUS-40 12.0 8.5 40.0(A) 27.6 45.0 Intercity Bus BUS-45 12.0 8.5 45.0(A) 25.5 45.0 City Transit Bus CITY-BUS 10.5 8.5 40.0(A) 24.5 42.0 Conventional School Bus S-BUS 36 10.5 8.0 35.8 23.8 38.9 Large School Bus S-BUS 40 10.5 8.0 40.0 25.4 39.4 Articulated Bus A-BUS 11.0 8.5 60.0(A) 21.3 39.8 Intermediate Semitrailer WB-40 13.5 8.0 45.5 19.3 40.0 Intermediate Semitrailer WB-50 13.5 8.5 55.0 17.0 45.0 Motor Home and Boat Trailer MHB 12.0 8.0 53.0 35.1 50.0 (A) Add 1.5 feet in length where buses are equipped with bicycle racks. Source: A Policy on Geometric Design (1). Table 4-3. Design vehicle dimensions for select vehicles.

AASHTO publishes templates that delineate minimum turn- ing radii and suggest paths for each design vehicle. These templates should be used to check the provision of adequate maneuvering space. Bus turns into and out of the busway will depend upon specific service design features. There may be circumstances where buses need to enter and leave busways at intersections, and suitable provisions should be made for these move- ments. However, the number of such locations should be kept to a minimum as discussed in Chapter 3. In addition, some intersections should be designated for emergency turn- ing movements. The Los Angeles Department of Trans- portation designated a few key intersections where operators can make turns to leave the busway in emergency situations. Design Year Designs for new transit and highway facilities are normally based on a minimum 20-year time horizon. However, roadway and intersection design improvements should have at least a 10-year horizon. Operational improvements, such as bus lanes or traffic signal changes, should have at least a 2- to 5-year horizon. Designs also should be assessed for “base year” conditions—conditions for the year that the intersection improvements and busway are placed in service. Design Volumes The design hourly volume (DHV) is the projected volume that is used for design. DHV is typically expressed as a per- centage of the expected average daily traffic. The 30th highest hour of the year is traditionally used for design. However, in urban and suburban areas, the morning and evening peak hours provide a sound basis for establishing intersection requirements and assessing intersection opera- tions. Volume should be obtained by 15-minute intervals, for each intersection movement, for each type of vehicle. 11 Characteristic 40-ft Regular Bus 45-ft Regular Bus 60-ft Articulated Bus Length 40 ft 45 ft 60 ft Width without mirror 8.2-8.5 ft (A) 8.5 ft (A) 8.5 ft (A) Height (to top of air conditioning) for design 9.9-11.5 ft (B) 12.5 ft (C) 11 ft (B) Overhang Front 7.2 ft 7.9 ft 8.8-8.9 ft Rear 9.3 ft 9.8 ft 8.6-9.7 ft Wheelbase (rear) 25 ft 22.9 ft 23.3-24.5 ft Driver's Eye Height 7 ft (C) 7 ft (C) 7 ft (C) Weight Curb Weight 27,000-28,200 lbs 38,150 lbs 38,000 lbs Gross Weight 36,900-40,000 lbs 55,200 lbs 66,600 lbs Ground to Floor Height 2.3 ft 2.3 ft 2.3 ft Passenger Capacity Seats 45-50 50 76 Standees (Crush Load) 20 28 38 Turning Radius Inside 24.5-30 ft 24.5-30 ft 27.3 ft Outside (D) 42-47 ft 42-47 ft 39.8-42 ft Outside with Overhang 45.5-51 ft 45.5-51 ft 44.3 ft Doors - Number (typical) 2 2 2-3 Width of each door 2.3-5 ft 2.5-5 ft 2.5-5 ft Angles (degrees) Approach 10° 10° 10° Breakover 10° 10° 10° Departure 9.5° 9.5° 9.5° (A) With mirrors envelope becomes 10 to 10.5 feet. (B) Use 16 feet as minimum governing design clearance. (C) Use 3.5 feet design. (D) Add 1.5 feet where buses are equipped with bicycle racks. Exact dimensions may vary by bus manufacturer. Source: TCRP Project D-09 Phase II Draft Guide (7). Table 4-4. Design characteristics for 40-, 45-, and 60-foot buses.

Design Speed Roadway and busway intersections and alignment features depend upon the designated design speed, i.e., the speed selected to establish the geometric features of the roadway. Design speed depends upon the functional class of the road- way, topography, and land use. It ranges from 20 to 70 mph in 10-mph increments. Design speeds for busways generally range from 40 to 50 mph,although lower speeds may be necessary in constrained environments. Arterial street and roadway speeds usually fall in this range, but may be as low as 30 mph in some situations. Capacity Considerations Busway intersections should provide sufficient roadway, walkway, and station capacities to serve anticipated demands and operate at reasonable levels of service. Detailed compu- tational procedures and guidelines are set forth in the High- way Capacity Manual (8) and the Transit Capacity and Quality of Service Manual (9). Both manuals base their analyses on peak 15-minute flow rates expressed in vehicles (or people) per hour. Both state that operating at maximum capacities results in long delays and poor reliability. Intersection Capacity The capacity of each intersection approach depends upon (1) the number and efficiency of each moving travel lane; (2) the nature and extent of interferences such as signal timing, cross- street requirements and left-turn conflicts with opposing traffic; and (3) the headways (or saturation flows) that reflect traffic composition and grades. Because each lane may perform differently, computations are best done on a lane-by-lane basis. Intersection Levels of Service Signalized intersection performance should be assessed in terms of the control delay that results from the red signal times and queues of traffic. This control delay depends upon the volume (demand) to capacity ratio and the red time per cycle. Levels of service (LOS) are measured in the amount of control delay and range from LOS A (less than 10 seconds per vehicle) up to LOS F (more than 80 seconds per vehicle). LOS C and D are the desired maximum service levels for urban and suburban conditions (up to 55 seconds per vehicle). A maximum volume to capacity ratio of 0.85 is also suggested as the upper limit of system adequacy. Transit Capacity Quality of service for transit passengers is defined as the overall measures or perceived performance of a transit service from the passenger’s point of view. It reflects what a potential passenger considers when deciding whether to use transit. The considerations include (1) whether transit service is available, and, (2) if available, how transit will compare with competing modes. Table 4-5 provides the framework for defining fixed- route and demand-responsive quality of service ratings. The actual passenger capacity of a transit route or stop depends upon the number of vehicles that can be processed and the number of people that can be served. It is measured along the way and at stops, terminals, and junctions near sta- tions (i.e., the critical locations that govern capacity). The highest achievable minimum headway along a route governs the number of transit vehicles or units that can be processed. Busway stops and stations normally govern the number of buses that can be accommodated along a busway. Typically, the passenger demand during the peak 15 min- utes at the maximum load section establishes the service fre- quency for a given loading standard. Then, whether this service frequency can be processed through the busiest points of passenger activity along the line needs to be determined. From a busway intersection perspective, it is essential to provide enough berths (loading positions) at each stop and to provide passing capabilities at stops or stations where space permits. Detailed computational procedures are contained in the Transit Capacity and Quality of Service Manual (9). 12 Transit Service Transit Stop Route Segment System Fixed-Route Measures Availability Frequency Hours of Service Service Coverage Comfort and Convenience Passenger Load Reliability Transit vs. Automobile Travel Time Demand-Responsive Measures Availability Response Time Span of Service Comfort and Convenience On-Time Performance Trips Not Served Transit vs. Automobile Travel Time Source: Transit Capacity and Quality of Service Manual (9). Table 4-5. Transit quality of service framework.

A general guide is to provide at least two berths for each direc- tion at busway stations. Pedestrian Capacity Levels Pedestrian service levels and capacities are key inputs into designing bus stops/stations, walkways, stairways leading to and from stations, and general pedestrian movements in the station and intersection influence areas. 13 Service levels for pedestrians using walkways are shown in Table 4-6. This table provides the speeds and flows for vari- ous units of effective sidewalk width. The maximum capac- ity given in the Transit Capacity and Quality of Service Manual (9) is 25 pedestrians per foot per minute (p/ft/min). However, few sidewalks in the United States and Canada have rates that exceed 15 pedestrians per foot per minute. All rates are based on the clear or effective width after deducting for obstructions. LOS Space per Person (ft2) Average Speed (ft/min) Flow per Unit Width (p/ft/min) Volume/ Capacity A ≥ 35 260 0–7 0.0–0.3 B 25–35 250 7–10 0.3–0.4 C 15–25 240 10–15 0.4–0.6 D 10–15 225 15–20 0.6–0.8 E 5–10 150 20–25 0.8–1.0 F < 5 <150 Variable Variable Source: Adapted from Transit Capacity and Quality of Service Manual (9). Table 4-6. Pedestrian levels of service on walkways.

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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.

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