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Chapter Seven Intersections

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Chapter Seven Intersections DelDOT Road Design Manual Chapter Seven Intersections The intersection of two or more roads pre- An important consideration in the design of sents an opportunity for conflict among vehi- intersections is the treatment of right-turn cles. For freeways, the potential for conflict is lanes. Right turns can be free flowing, yield or significantly reduced through the use of inter- stop controlled. In order to operate properly, changes. But interchanges usually are not fea- free flowing right-turn lanes need to have an sible for the vast majority of intersections on adequate acceleration distance free of access arterials and collectors. This chapter is a gen- points for drivers to safely merge into the eral discussion of intersection design with through traffic. Some drivers, particularly those elements of particular application to this older drivers, are apprehensive when entering state. The details on intersection design are another leg of an intersection and may stop or found in Chapter 9 of AASHTO’s Green slow down in the merge lane until the lane is Book. clear of traffic. However, when properly de- signed, the majority of drivers will use the The principal objectives in the design of at- lane as proposed. grade intersections are: • To minimize the potential for and se- verity of conflicts, 7.1 GENERAL • To provide adequate capacity, and CONSIDERATIONS • To assure the convenience and ease of This section describes the various types of drivers in making the necessary ma- intersections and the general criteria that must neuvers. be considered during design. Project intersec- In the design of intersections there are three tion design configurations are developed dur- elements to consider: ing the project development phase based upon (1) Perception-reaction distance, capacity analysis, accident studies, pedestrian use, bicycle use and transit options. In addi- (2) Maneuver distance, and tion, design-hour turning movements, size and (3) Queue-storage distance. operating characteristics of the predominant The distance traveled during the perception- vehicles, types of movements that must be reaction time varies with vehicle speed, driver provided, vehicle speeds, and existing and alertness, and driver familiarity with the loca- proposed adjacent land-use are considered. tion. Where left-turn lanes are introduced, this distance includes that to brake and change Intersection designs range from a simple lanes. Where no turn lanes are provided, the residential driveway to a complicated conver- distance needed is for the driver to brake com- gence of several high-volume multi-lane fortably. The storage length should be suffi- roadways. They all have the same fundamental cient to accommodate the longest queue most design elements: (1) level of service, (2) commonly experienced. alignment, (3) profile, (4) roadway cross sec- tion(s), and (5) sight distance. However, other November 2006 Intersections 7-1 elements are introduced in intersection designs 7.1.3 ALIGNMENT such as: speed-change lanes, turning lanes, auxiliary lanes, traffic islands, medians (flush Ideally, intersecting roads should meet at, and raised), channelization, pedestrian and or nearly at, right angles. Roads intersecting at bicycle accessibility, and traffic signalization. acute angles require extensive turning road- way areas and tend to limit visibility, particu- When identified in the project scope of larly for drivers of trucks. Acute-angle inter- work, traffic calming measures may be a part sections increase the exposure time of vehicles of an intersection design. The Department’s crossing the main traffic flow and may in- Traffic Calming Design Manual gives details crease the accident potential. Although a right on the alternative treatments and general de- angle crossing normally is desired, some de- sign guidance including those for roundabouts. viation is permissible. Angles above approxi- Additional information on the design of mately 60 degrees produce only a small reduc- roundabouts can be found in FHWA’s publi- tion in visibility, which often does not warrant cation Roundabouts: An Informational Guide. realignment closer to 90 degrees. Intersections on sharp curves should be 7.1.1 TYPES OF INTERSECTIONS avoided wherever possible because the su- The three basic types of intersections are perelevation of pavements on curves compli- the three-leg or T-intersection (with variations cates the design of the intersection. Also, this in the angle of approach), the four-leg inter- situation often leads to sight distance problems section, and the multi-leg intersection. Each because of the sharp curve. It may be desirable intersection can vary greatly in scope, shape, to flatten the curve, or to introduce two curves use of channelization and other types of traffic separated by a tangent through the intersec- control devices. The simplest and most com- tion. If either of these options is used, a sub- mon T-intersection is the private entrance or stantial change in alignment may be necessary. driveway. At the other extreme, a major high- way intersecting another major highway usu- 7.1.4 PROFILE ally requires a rather complex design. Combinations of grade lines that make ve- hicle control difficult should be avoided at 7.1.2 LEVELS OF SERVICE intersections. The grades of intersecting high- Levels of service for highway facilities ways should be as flat as practical on those were discussed in Chapter Two. The relation- sections that are to be used as storage space ships between traffic volumes and highway for stopped vehicles. Most vehicles must have capacity, together with operating speeds, pro- the brakes applied to stand still unless they are vide a measure of the level of service. The stopped on a gradient flatter than 1 percent. characteristics of at-grade intersections can Grades in excess of 3 percent generally should have a dramatic effect on capacity and the be avoided in the vicinity of intersections. level of service. The profile grade lines and cross section on Capacity analysis is one of the most impor- the legs of an intersection should be adjusted tant considerations in the design of intersec- for a distance back from the intersection to tions. Optimum capacities and improved con- provide a smooth junction and adequate drain- ditions can be obtained when at-grade inter- age. Normally, the grade line of the major sections include auxiliary lanes, proper use of highway should be carried through the inter- channelization, and traffic control devices. section, and that of the crossroad should be The Highway Capacity Manual provides the adjusted to it. This design requires transition procedures for analyzing the capacity of sig- of the crown of the minor highway to an in- nalized and unsignalized intersections. clined cross section at its junction with the major highway. For intersections with traffic 7-2 Intersections July 2004 DelDOT Road Design Manual signals, or where signals may be warranted in It may be necessary to impose turn restrictions the near future, it may be desirable to warp the at some locations, prohibit pedestrian cross- crowns of both roads to avoid a pronounced ings, or provide frontage roads for access to hump or dip in the grade line of the minor intersecting roads. Where crossroads are highway. Intersections in superelevation areas widely spaced each at-grade intersection must are difficult to provide smooth grades or ade- necessarily accommodate all cross, turning quate drainage for and should be avoided. and pedestrian movements. 7.1.5 FRONTAGE ROAD 7.2 TURNING MOVEMENTS INTERSECTIONS All intersections involve some degree of ve- When a divided arterial highway is flanked hicular turning movements. There are various by a frontage road, the problems of design and factors that influence the geometric design of traffic control are more complex. Four sepa- turning lanes. The design controls for turning rate intersections actually exist at each cross roadways are the traffic volume and types of street. vehicles making the turning movement. The roadway of primary concern is that used by The problem becomes more severe when right-turning traffic but may also be used for the distance between the arterial and frontage other roadways within the intersection. Figure road is relatively small. Generally, the outer 7-1 shows the terminology used when design- separation between the two roadways should ing turning movements. The outer trace of the be 150 ft [50 m] or more. front bumper overhang and the path of the in- ner rear wheel establish the boundaries of the Quite often, right-of-way considerations turning paths of a design vehicle. make it impractical to provide the full desired outer separation width. The alternative is to The three typical types of designs for right- accept a narrow outer separation between turning roadways in intersections are: cross roads and design a bulb-shaped separa- tion in the immediate vicinity of each cross (1) A minimum edge-of-traveled-way design road. (Green Book, pages 583 to 621), (2) A design with a corner triangular island (Green Book, pages 634 to 639), and 7.1.6 DISTANCE BETWEEN INTERSECTIONS (3) A free-flow design using simple radius or compound radii (Green Book, pages 639 Criteria for location, frequency and layout to 649). The turning radii and pavement of private entrances and driveways are docu- cross slopes for free-flow right turns are mented in DelDOT’s Standards and Regula- functions of design speed and type of ve- tions for Access to State Highways. Illustrative hicle. sketches are shown for typical entrance and driveway designs for various conditions. For other types of public intersections, there are no fixed criteria as to frequency or distance be- tween intersections. However, intersection spacing should provide sufficient distance to allow the proper development of all necessary turning lanes, bypass lanes, and, if signalized, proper signal coordination. Ideally this dis- tance should be at least 350 ft [110 m] or more. Where intersections are closely spaced, several considerations should be kept in mind.
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