Chapter 6 Design of Highway At-Grade Intersections

1. Chapter 6Design of Highway At-Grade Intersections • typical intersection types and their characteristics • general design principles for at-grade intersections • Know how to find minimum radii for the curves at at-grade intersections • Know what the channelization is, objectives for using channelization, and factors affecting its application

2. Intersection types At-grade Grade separated With ramps (Interchanges) Without ramps (meaning no connection between the intersection roads!)

3. Topics to be covered • Types of intersections • Objectives in the design of at-grade intersections • Alignment considerations • Grade considerations • Curve types and minimum radii • Channelization: what it is, why we need it, how we provide it

4. At-grade intersections (pay attention to channelization techniques): T or 3-leg

7. Speed change lanes

9. At-grade intersections (pay attention to channelization techniques): 4-leg

10. At-grade intersections (pay attention to channelization techniques): Multi-leg Mulry Square in Greenwich Village, Manhattan, New York City

11. Rotary Intersections (Roundabouts) A rotary intersection is one in which all traffic merges into and emerges from a one- way road around a central island. There is little delay to traffic due to speed reductions and no delay at all due to stopping. Its advantages include: 1. Continuos traffic movement from all legs; 2. Accidents are likely to be less serious; 3. Where more than 4-legs are involved , the design layout may be simplified; 4. The cost of this type may be considerably less than that of grade separation structures.

12. Disadvantage include: 1. Requires large area; 2. Costs are more than other at-grade intersections; 3. It is not suitable for large pedestrians movements; 4. There is little delay due to speed reduction; 5. It needs long weaving sections to ensure smooth flow.

14. Roundabouts – different from circles Typical markings for roundabouts with one lane • Circles may have a signal, STOP signs, no-control at entry – Roundabouts are always controlled by Yield signs. • Splitter islands • Peds are not allowed to use the central island • No parking in the circle • Circulating vehicles always have the ROW.

15. Roundabouts – different from circles • Advantages: • Provides non-stop movements • Reduce crash occurrences • Reduce crash severity • Esthetically appealing • Function as a traffic calming measure In Australia Good for low to medium traffic. Definitely NOT for high volume intersections  Too many weavings In Maryland In Norway

16. At-grade intersection design objectives and considerations • Need to meet two conflicting objectives: • Minimize the severity of potential conflicts among different streams of traffic and between pedestrians and turning vehicles. • Provide for the smooth flow of traffic across the intersection Adequate pavement width and approach sight distances must be provided. Operating characteristics of both the vehicles and pedestrians

17. At-grade intersection design considerations • Alignment and profile design • Angle of intersecting roads • Suitable channelization system for the traffic pattern • Minimum required widths of turning roadways • Adequate sight distance for the type of traffic control used (no control, Yield, Stop, Signal)

18. Suggested improvements to intersections with acute “angle” problems Alignment Longer walking distance Angle of turn Obtuse angle The angle of turns should be 60 to 120 degrees. Acute angle Superelevation problem Wider pavement needed for turning vehicles

19. Profile • Make it as flat as possible • Avoid approach grades in excess of 3% • Avoid grade changes at intersections (Crest  sight distance problems, Sag  drainage problems) • The grade line of the major highway should be carried through the intersection. Adjust the grade for the normal crown of the crossroad to an inclined cross section at its junction with the major road.  A good example at Columbia Lane and Grandview Road (the signalized intersection on Columbia Lane just north of DI).

20. Curves at at-grade intersections Rule 1: When the turning speed at an intersection is assumed to be 15 mph or less, the curves for the pavement edges are designed to conform to at least the minimum turning path of the design vehicle. If the speed is greater than this, the design speed is also considered to determine the radius (Remember? R = u2/(g[e + fs]).  This means that you are not supposed to use the values in Table 7-2 and 7-3 in the textbook. Rule 2: The angle of intersection affects the curve design. • Typical types: • Simple curve • Simple curve with taper • 3-centered compound curve

21. Three typical curve design methods

22. Minimum edge of pavement designs: Simple curve and simple curve with taper (Table 7.2) R

23. Channelization: objectives • Direct the paths of vehicles • Control the merging, diverging, and crossing angle of vehicles • Reduce the amount of paved area • Provide a clear indication of the proper path for different movements • Give priority to the predominant movements • Provide pedestrian refuge • Provide separate storage lanes for turning vehicles • Provide space for traffic control devices for visibility • Control prohibited turns • Separate different traffic movements at signalized intersections with multiple-phase signals • Restrict the speeds of vehicles

24. Channelization: considerations • Motorists should not be required to make more than one decision at a time. • Sharp reverse curves and turning paths greater than 90 degrees should be avoided. • Merging and weaving areas should be as long as possible, but other areas of conflict between vehicles should be reduced to a minimum • Crossing traffic streams that do not weave or merge should intersect at 90 degrees, although a range of 60-120 degrees is acceptable • The intersecting angle of merging streams should be such that adequate sight distance is provided. • Refuge areas for turning vehicles should not interfere with the movement of through vehicles • Prohibited turns should be blocked wherever possible • Decisions on the location of essential traffic control devices should be a component of the design process.

25. Chapter 7Grade Separations- and interchanges Intersections Although the construction cost of the grade separated structures is very high, but the following advantages justify its uses: 1. To ensure free movement of high speed traffic on motorways; 2. Increase capacity lane to its initial value; 3. Increase safety for all vehicles;

26. 4. More economic in operation and losses and cheaper in expensive areas; 5. Save time and money and more comfort; 6. It can be implemented at any site.

27. The separated at-grade intersections have the following disadvantages: 1. High construction costs and maintenance; 2. Not suitable for five or more legs intersections; 3.The layout of grade separation may be confusing to some drivers .

28. Interchanges can be divided into the following main types: a) 3- way interchanges: may be a T -or Y types intersection as shown in Fig.34. Both types utilize a single bridge structure and are suitable for expressway provided that the loop movement is relatively small. In the case of heavy movement, an extra bridge may be constructed so that both turning movements are favored equally.

29. b) 4- way interchanges: the simplest type of 4-way interchanges is the diamond, which consisting of a single bridge and four way ramps. This type can be located within relatively narrow land area. It has high speed entrance and exit ramps on the main road and at -grade ramp terminals on the minor road.

30. c) The Cloverleaf : It is only 4- way single structure interchange having no terminal left turns at-grade. It is the most common form of interchange and is required as the ultimate answer to intersection problems. It being very uncomplicated to use with

31. Examples of grade separated interchanges

32. Examples of grade separated interchanges (cont)

33. More examples of directional interchanges