1. 1 At-Grade Intersection Design

2. 2 Objectives Define general intersection design objectives Describe factors that influence curb return design For a nice overview of intersection design, see http://www.fhwa.dot.gov/environment/flex/ch08.htm http://www.fhwa.dot.gov/environment/flex/ch08.htm

3. 3 Intersections Grade-separated without ramps Interchanges (grade separated with ramps) At-grade

4. 4 Intersection – Design Controls Primary controls: Functional class of roadways Topography and environment (manmade and natural) Secondary controls: Design speed Design vehicles Traffic Characteristics (design volumes, level of service)

5. 5 Intersection Design Elements/Considerations 4 or fewer legs (within functional area) As close to 90 degrees as possible Approach (flat and straight as possible) Avoid > 6% on low speed ( 3% on high speed (≥ 50 mph) Provide min. grades and max. vertical curve lengths

6. 6 Intersection Design Elements/Considerations Continue grade throughout the intersection Make adjustments away from intersection Traffic lanes should be visible and obvious to motorists Motorists should understand the path they are supposed to take Horizontal and vertical alignment should facilitate frequent braking

7. 7 Types of Intersection 4-leg T-intersection 5 or more legs Traffic circles Rotaries: large diameter > 300 ft, allows speeds > 30 mph with minimum horizontal deflection of the path of through traffic Neighborhood traffic circle: small diameter, for local streets, traffic calming Roundabout Yield control at each approach Separation of conflicting movements Speed < 30 mph (typically)

8. 8 Basic Principles Minimize severity of potential conflicts Provide for smooth flow of traffic Consider both vehicles and pedestrians

9. 9 Elements of Design Design of alignment Design of channeling system Determination of minimum required widths of turning roadways Speeds > 15 mph Intersection sight distance Determination of number of lanes Provision of turning lanes

10. 10 Alignment Horizontal 90° intersection of approaches Skewed Visibility Longer crossing times in some cases

11. 11 Profile (Vertical) Should facilitate driver’s control of vehicle Avoid significant changes in grade Typically ≤ 3% Continue major street grade through intersection

12. 12 Curb Radius Factors of influence: Design vehicle Intersection angle Approach width and parking Channelization Pedestrians Allowable speed reduction

13. 13 Curb Radius Green Book Minimums assume: Turning Speed LTE 10 mph Vehicle properly positioned beginning and end Vehicle doesn’t cross inner edge of pavement

14. 14 Design Vehicle Source: www\fhwa\Flexibil ity in Highway Design - Chapter 8 - FHWA.htm

15. 15 Radius Design Simple curve Low speed collector, local streets Simple circle with taper 3-centered compound curve

16. 16 Minimize lane encroachment R = 15 feet

17. 17 Simple Curve (passenger car template)

18. 18 Simple Curve with Taper (passenger car template)

19. 19 Compound curves (passenger car template)

20. 20

21. 21

22. 22 Curb Radius General Guidance 10 to 25 ft. local 25 to 30 ft. collectors 30 to 35 ft. unchannelized intersections with arterials

23. 23 Source: www\fhwa\Flexibility in Highway Design - Chapter 8 - FHWA.htm

24. 24 High Speed Turns

25. 25 Channelization Separates conflicting movements into definite paths of travel Uses pavement markings or traffic islands Directs vehicle paths so no more than 2 paths cross at one point Controls merging, diverging, and crossing angle of vehicles Decreases vehicle wander

26. 26 Channelization Provides clear path for different movements Gives priority to dominant movements Provides pedestrian refuge Provides storage area for turning vehicles Controls prohibited turns Restricts speed

27. 27 Types of Channelization Raised islands Urban Provides refuge for pedestrians <= 50 ft 2 in urban areas <= 75 ft 2 in rural areas Pavement markings Low pedestrian volume, low approach speeds

28. 28 Types of Channelization Pavement edge Rural – painted if high speed Formed by diverging through and right turn lanes

29. 29 Delineation With Pavement Marking

30. 30 Left & Right Turn Lane Warrants Turning movement volumes Accident experience Capacity Source: www\fhwa\Flexibility in Highway Design - Chapter 8 - FHWA.htm

31. 31 Left & Right Turn Lane Design Number of likely queued vehicles Type of control Number of turning vehicles Length of vehicles

32. 32 Deceleration Lanes Provides distance for turning vehicles to decelerate away without interfering with through traffic Deceleration lane length depends on: Speed number of queued vehicles vehicle length

33. 33 Median Openings …(see GB for Design specs)

34. 34 Intersection Design Considerations for Pedestrians

35. 35

36. 36 Provide median refuge to provide crossing in stages

37. 37 Provision of crosswalks

38. 38 Intersections on High Speed Rural Divided Expressways

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