AASHTO a policy on geometric design of highways and streets 2004 part 2

CHAPTER INTERSECTIONS INTRODUCTION An intersection is defined as the general area where two or more highways join or cross, including the roadway and roadside facilities for traffic movements within the area Each highway radiating from an intersection and forming part of it is an intersection leg The most common intersection at which two highways cross one another has four legs It is not recommended that an intersection have more than four legs Intersections are an important part of a highway facility because, to a great extent, the efficiency, safety, speed, cost of operation, and capacity of the facility depend on their design Each intersection involves through- or cross-traffic movements on one or more of the highways and may involve turning movements between these highways Such movements may be facilitated by various geometric design and traffic control, depending on the type ofintersection The three general types of highway crossings are at-grade intersections, grade separations without ramps, and interchanges This chapter deals primarily with the design of intersections at grade; the latter two intersection types are discussed separately in Chapter lO Certain intersection design elements, primarily those concerning the accommodation of turning movements, are common and applicable to intersections and to some parts of certain interchanges The design elements in the following discussions apply to intersections and their appurtenant features GENERAL DESIGN CONSIDERATIONS AND OBJECTIVES The main objective of intersection design is to facilitate the convenience, ease, and comfort of people traversing the intersection while enhancing the efficient movement of motor vehicles, buses, trucks, bicycles, and pedestrians Intersection design should be fitted closely to the natural transitional paths and operating characteristics of its users Five basic elements should be considered in intersection design • Human Factors Driving habits Ability of drivers to make decisions Driver expectancy Decision and reaction time Conformance to natural paths of movement Pedestrian use and habits Bicycle traffic use and habits • Traffic Considerations Design and actual capacities 555 AASHTD Geometric Design ofHighways and Streets Design-hour turning movements Size and operating characteristics of vehicle Variety of movements (diverging, merging, weaving, and crossing) Vehicle speeds Transit involvement Crash experience Bicycle movements Pedestrian movements • Physical Elements Character and use of abutting property Vertical alignments at the intersection Sight distance Angle of the intersection Conflict area Speed-change lanes Geometric design features Traffic control devices Lighting equipment Safety features Bicycle traffic Environmental factors Cross walks • Economic Factors Cost of improvements Effects of controlling or limiting rights-of-way on abutting residential or commercial properties where channelization restricts or prohibits vehicular movements Energy consumption • Functional Intersection Area An intersection is defined by both its functional and physical areas (1), as illustrated in Exhibit 9-1 The functional area of an intersection extends both upstream and downstream from the physical intersection area and includes any auxiliary lanes and their associated channelization The functional area on the approach to an intersection or driveway consists of three basic elements: (l) perception-reaction distance, (2) maneuver distance, and (3) queue-storage distance These elements are shown in Exhibit 9-2 The distance traveled during the perception-reaction time will depend upon vehicle speed, driver alertness, and driver familiarity with the location Where there is a left-or right-tum lane, the maneuver distance includes the length needed for both 556 Intersections OEtlNEO BY P»YSICAl AREA DEFINED ay FUNCTIONAL INTERSECTION AREA Exhibit 9-1 Physical and Functional Intersection Area J GO GO Beoin Be9,nPerceJ>tion Deceleration Reocti-m L VI II> II> e L U Exhibit 9-2 Elements of the Functional Area of an Intersection 557 AASHTQ.-Oeometric Design ofHighways and Streets braking and lane changing In the absence of tum lanes, it involves braking to a comfortable stop The storage length should be sufficient to accommodate the longest queue expected most of the time Ideally, driveways should not be located within the functional area of an intersection, as described above and shown in Exhibit 9-1, or in the influence area of an adjacent driveway TYPES AND EXAMPLES OF INTERSECTIONS General Considerations The basic types of intersections are the three-leg or T, the four-leg, and the multileg At each particular location, the intersection type is determined primarily by the number of intersecting legs, the topography, the character of the intersecting highways, the traffic volumes, patterns, and speeds, and the desired type of operation Any of the basic intersection types can vary greatly in scope, shape, and degree of channelization Once the intersection type is established, the design controls and criteria discussed in Chapter and the elements of intersection design presented in Chapter 3, as well as in this chapter, should be applied to arrive at a suitable geometric plan In this section each type of intersection is discussed separately, and likely variations of each are shown It is not practical to show all possible variations, but those presented are sufficient to illustrate the general application of intersection design Many other variations of types and treatment may be found in the NCHRP Report 279, Intersection Channelization Design Guide (2), which shows detailed examples that are not included in this policy Although many of the intersection design examples are located in urban areas, the principles involved apply equally to design in rural areas Some minor design variations occur with different kinds of traffic control, but all of the intersection types shown lend themselves to cautionary or non-stop control, stop control for minor approaches, four-way stop control, and both fixed-time and traffic-actuated signal control Right turns without stop or yield control are sometimes provided at channelized intersections Such free-flow right turns should be used only where an adequate merge is provided Where motor vehicle conflicts with pedestrians or bicyclists are anticipated, provisions for pedestrians and bicycle movements must be considered in the design In built-up areas, the use of free-flow right-tum lanes should be considered only where significant traffic capacity or safety problems may occur without them and adequate pedestrian crossings can be provided Simple intersections are presented first, followed by more complex types, some of which are special adaptations In addition, conditions for which each intersection type may be suited are discussed below 558 Intersections Three-Leg Intersections Basic Types of Intersections Basic forms of three-leg or T intersections are illustrated in Exhibits 9-3 through 9-8 The most common type of T intersection is shown in Exhibit 9-5A The normal pavement widths of both highways should be maintained except for the paved returns or where widening is needed to accommodate the selected design vehicle This type of unchannelized intersection is generally suitable for junctions of minor or local roads and junctions of minor roads with more important highways where the angle of intersection is not generally more than 30 degrees from Exhibit 9-3 Channelized High-Type "T" Intersections til Exhibit 9-4 Three-Leg Rural Intersection, Channelized "T" 559 AASHTG-Geometric Design ofHighways and Streets - PLAIN '1' INTERSECTION -A- - 'T'INTERSECTION (With Right Tum lone) - BExhibit 9-5 "T" Intersections perpendicular (i.e., from approximately 60 to 120 degrees) In rural areas, this intersection type is usually used in conjunction with two-lane highways carrying light traffic In suburban or urban areas, it may be satisfactory for higher volumes and for multilane roads Where speeds and turning movements are high, an additional area of surfacing or flaring may be provided for maneuverability, as shown in Exhibit 9-5B and 9-6 560 Intersections ~- - - - - - -~- - '1' INTERSECTION (WITH RIGHT HAND PASSING lANE) -A- 'T' INTERSECTION (WITH RIGHT HAND PASSING LANE AND RIGHT TURN LANE) - B- Exhibit 9-6 Channelized "T" Intersections The use of auxiliary lanes, such as left- and right-tum lanes, increases capacity and creates better operational conditions for turning vehicles Left turns from the through highways are particularly difficult because a vehicle must slow down and perhaps stop before completing the turn Intersections with separate left-tum lanes permit following-through vehicles to maneuver around these slower turning vehicles Existing intersections can have an auxiliary lane added with minimal difficulties to provide the intersection types shown in Exhibits 9-5B and 9-6 561 AASHTO-Geometric Design ofHighways and Streets tIf J - I'~ -_ ::::;:::;;-_ •-: .L+_.·.;- ~~[...]... configuration that is appropriate, except at a minor crossroad, for an intersection on a two-lane highway operating near capacity or carrying moderate volumes at high speeds The two-lane approach on the major highway can be converted to a four-lane section with a divisional island The additional areas are used for speed changes, maneuvering, and storage of turning vehicles The form of channelization on. .. intersections with signal control An island marked on the pavement is not as positive a separator as a curbed divisional island, but it is appropriate where sand or snow may be a maintenance problem and where any curbed island may be an obstruction, as on high-speed rural highways Channelized Four-Leg Intersections The usual configurations of four-leg intersections with simple channelization are shown... have high superelevation rates and where the minor-road approach has adverse grades and a sight distance restriction due to the grade line 581 AASHTO- Geometric Design ofHighways and Streets Profile Combinations of grade lines that make vehicle control difficult should be avoided at intersections Substantial grade changes should be avoided at intersections, but it is not always practical to do so Adequate... whenever practical because of the difficulty in adjusting grades to provide a suitable crossing Gradelines of separate turning roadways should be designed to fit the cross slopes and longitudinal grades of the intersection legs The alignment and grades are subject to greater constraints at or near intersections than on the open road At or near intersections, the combination of horizontal and vertical alignment... achieving an optimal capacity and operational balance When the capacity needs can only be met by increasing the entry width, this can be accomplished in two ways: (1) by adding a full lane upstream of the roundabout and maintaining parallel lanes through the entire entry, or (2) by widening the approach gradually (flaring) through the entire entry An example of entry flaring in two quadrants of a roundabout... this chapter as it relates to the various elements of intersection design Optimum capacities and levels of service can be obtained when intersections include auxiliary lanes, appropriate channelization, and traffic control devices For more complete discussion of capacity and level -of- service analysis for intersections, including operational analysis procedures, refer to the Highway Capacity Manual (HCM)... and in controlling their operations Site conditions generally establish definite alignment and grade constraints on the intersecting roads It may be practical to modify the alignment and grades, however, in order to improve traffic operations 579 AASHTo- Geometric Design ofHighways and Streets Alignment Regardless of the type of intersection, for safety and economy, intersecting roads should generally... volumes of traffic and the type of traffic control used, flaring of the intersecting roadways can be accomplished by parallel auxiliary lanes, as on the highway shown horizontally, or by pavement tapers, as shown on the crossroad Flaring generally is similar on opposite legs Parallel auxiliary lanes are essential where traffic volume on the major highway is near the uninterrupted-flow capacity of the... along each of the major roadways may be desirable This design may be used where a grade separation is not practical, as in flat terrain with traffic having a high volume of heavy trucks, or where it is desired to defer the construction of a grade separation Where movements in the other quadrants reach the proportions of through movements, additional diagonal roadways might be provided, but with major... traffic lanes that are clearly visible to drivers at all times, clearly understandable for any desired direction of travel, free from the potential for conflicts to appear suddenly, and consistent in design with the portions of the highway just traveled The combination of vertical and horizontal curvature should allow adequate sight distance at an intersection As discussed in Chapter 3, "Combinations ... upstream and downstream from the physical intersection area and includes any auxiliary lanes and their associated channelization The functional area on the approach to an intersection or driveway... example, separate turning lanes and divisional islands may be used, as appropriate, to fit the particular situation 573 AASHTO- Geometric Design ofHighways and Streets Modern Roundabouts A recent... intersections with signal control An island marked on the pavement is not as positive a separator as a curbed divisional island, but it is appropriate where sand or snow may be a maintenance problem and