Intersections & Interchanges ( Highway Engineering Dr. Sherif El-Badawy )
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This document discusses different types of intersections and interchanges, including their key elements and design considerations. It describes at-grade intersections, channelization techniques, and different interchange configurations like diamonds, cloverleafs, and trumpets. The document also covers factors in curb radius, turn lane, and sight distance design, and provides examples of determining sufficient sight distance at intersections.
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Intersections & Interchanges ( Highway Engineering Dr. Sherif El-Badawy )
- 1. 10/23/2012 1 Intersections & Interchanges Intersections Types & Definition • Grade‐separated without ramps • Interchanges (grade separated with ramps) • At‐grade Intersection: Two or more streets join or cross at‐grade. The intersection includes the areas needed forThe intersection includes the areas needed for all modes of travel: pedestrian, bicycle, motor vehicle, and transit.
- 2. 10/23/2012 2 At‐grade intersections • All highways have intersections at grade except freeways so that the intersection areaexcept freeways, so that the intersection area is a part of every connecting road or street. • In this area, crossing and turning movements occur. • Some intersection are channelized – minimize traffic accidents, speed control, prevention of prohibited turns, refuge may be provided for pedestrians. At‐grade Intersections Types Unchannelized T Unchannelized YUnchannelized Y Flared T
- 3. 10/23/2012 3 3‐leg intersections Y with turning roadways Unchannelized Channelized • Traffic circles –Rotaries: large diameter > 300 ft, allows At‐grade Intersections Types (Cont.) 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)
- 4. 10/23/2012 4 Intersection: Key Elements • Safety and efficiency • Consider both vehicles and pedestrians • Minimize severity of potential conflicts • In general, these conflicts may be classified as: M i fli t Merging Diverging Basic Principles – Merging conflicts • Occurs when vehicles enter a traffic stream – Diverging conflicts • Occurs when vehicles leave the traffic stream – Weaving conflicts • Occurs by merging then diverging – Crossing conflicts • Occurs when they cross paths directly Weaving Crossing
- 5. 10/23/2012 5 Crossing Conflicts Solutions • Time‐sharing • Space‐sharing • Grade separation (Interchanges) INTERCHANGES • Are classified according to the way they h dl l ft t i t ffihandle left‐turning traffic. INTERCHANGE CONFIGURATION ‐ are selected on the basis of structural cost, right of way costs and ability to serve trafficright‐of‐way costs, and ability to serve traffic.
- 6. 10/23/2012 6 Diamond Interchange Diamond Interchange • Diamond Interchangeg – Employ diamond ramps which connect to the cross road by means of an at grade intersection. – Left turns are accomplished by having vehicles turn left across traffic on the cross road.road.
- 7. 10/23/2012 7 Cloverleaf Interchange Cloverleaf InterchangeCloverleaf Interchange Cloverleaf InterchangeCloverleaf Interchange Employ Employ loop rampsloop ramps, in , in which vehicles turn left which vehicles turn left by turning 270 degrees by turning 270 degrees to the right.to the right.
- 11. 10/23/2012 11 ON‐RAMP (entrance to highway) ON‐RAMP (entrance to highway) OFF‐RAMP (exit to highway) OFF‐RAMP (exit to highway) Intersection – Design Controls Functional class of roadways Topography and environment (manmade and natural) Design speed Design vehicles Traffic Characteristics (design volumes, level of 22 Traffic Characteristics (design volumes, level of service)
- 12. 10/23/2012 12 Intersection Design 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 (< 40 mph) and > 3% on high speed (≥ 50 mph) • Provide min. grades and max. vertical curve lengths • Make adjustments away from intersection 23 • Traffic lanes should be visible and obvious to motorists • Motorists should understand the path they are supposed to take Elements of Design • Design of alignment D i f h li t• Design of channeling system • Determination of minimum required widths of turning roadways – Speeds > 15 mph • Intersection sight distance 24 • Determination of number of lanes – Provision of turning lanes
- 13. 10/23/2012 13 Alignment Horizontal • 90° intersection of approaches • Skewed – Visibility 25 – Longer crossing times in some cases Profile (Vertical) • Should facilitate driver’s control of vehicle • Avoid significant changes in grade • Typically ≤ 3% • Continue major street grade through intersection 26
- 14. 10/23/2012 14 Curb Radius Design • Factors: – Design vehicle – Intersection angle – Approach width and parking – Channelization 27 – Pedestrians – Allowable speed reduction Design Vehicle 28 Source: wwwfhwaFlexibility in Highway Design - Chapter 8 - FHWA.htm
- 15. 10/23/2012 15 Radius Design • Simple curve –Low speed collector, local streets • Simple circle with taper d d 29 • 3‐centered compound curve Minimize lane encroachment R = 15 feet 30
- 19. 10/23/2012 19 fh Fl b l H h D 37 Source: wwwfhwaFlexibility in Highway Design - Chapter 8 - FHWA.htm High Speed Turns 38
- 20. 10/23/2012 20 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 • Provides clear path for different movements 39 • Provides pedestrian refuge • Provides storage area for turning vehicles • Controls prohibited turns • Restricts speed Types of Channelization • Raised islands – Urban – Provides refuge for pedestrians – <= 50 ft2 in urban areas – <= 75 ft2 in rural areas P t ki 40 • Pavement markings – Low pedestrian volume, low approach speeds
- 21. 10/23/2012 21 Types of Channelization (Cont.) • Pavement edge Rural painted if high speed– Rural – painted if high speed – Formed by diverging through and right turn lanes 41 Delineation With Pavement Marking 42
- 22. 10/23/2012 22 Left & Right Turn Lane Warrants • Turning movementTurning movement volumes • Accident experience • Capacity 43 Left & Right Turn Lane Design • Number of likely queued vehiclesy q – Type of control – Number of turning vehicles – Length of vehicles 44
- 23. 10/23/2012 23 Deceleration Lanes • Provides distance for turning vehicles to d l t ith t i t f i ithdecelerate away without interfering with through traffic • Deceleration lane length depends on: – Speed – number of queued vehicles 45 number of queued vehicles – vehicle length Auxiliary Lanes Tapers MEDIAN MEDIAN
- 26. 10/23/2012 26 Adequate Sight Distance – ISD Allow drivers to have an unobstructed view of intersection D fi iti R i d ISD i th l th f d• Definition: Required ISD is the length of cross road that must be visible such that the driver of a turning/crossing vehicle can decide to and complete the maneuver without conflict with vehicles approaching the intersection on the cross road. 51 Adequate Sight Distance – ISD Sight Triangle – area free of obstructions necessary to complete maneuver and avoid collision – needed for approach and departure (from stop sign forfor approach and departure (from stop sign for example) – Exhibit 9‐50 Allows driver to anticipate and avoid collisions Allows drivers of stopped vehicles enough view of the intersection to decide when to enter 52
- 27. 10/23/2012 27 Sight Triangle area free of obstructions necessary to l t d id lli icomplete maneuver and avoid collision – needed for approach and departure (from stop sign for example) Consider horizontal as well as vertical, object below driver eye height may not be an 53 y g y obstruction AASHTO assumes 3.5’ above roadway 54
- 28. 10/23/2012 28 Sight Distance Obstruction Hidd V hi lHidden Vehicle 55 ISD Cases • No control: vehicles adjust speed • Stop control: where traffic on minor roadway must• Stop control: where traffic on minor roadway must stop prior to entering major roadway • Yield control: vehicles on minor roadway must yield to major roadway traffic • Signal control: where vehicles on all approaches are required to stop by either a stop sign or traffic signal 56 • All way stop • Stopped major roadway left‐turn vehicles – must yield to oncoming traffic
- 29. 10/23/2012 29 Case A– No Control • Rare? – Not really ‐ Iowa • Minimum sight triangle sides = distance traveled in 3• Minimum sight triangle sides = distance traveled in 3 seconds (design or actual?) = 2 seconds for P/R and 1 second to actuate brake/accel. • Assumes vehicles slow ~ 50% of midblock running speed 57 Case A– No Control • Prefer appropriate SSD on both approaches (minimum really)(minimum really) • Provided on lightly traveled roadways • Provide control if sight triangle not available • Assumes vehicle on the left yields to vehicle on the right if they arrive at same time 58
- 30. 10/23/2012 30 59 60 Critical speed is set to stopping distance dCritical speed is set to stopping distance dbb = a __= a __ddaa____ ddaa -- bb
- 31. 10/23/2012 31 Large Tree 72’ Example 25 mph 47’ 45 mph 61 45 mph Is sufficient stopping sight distance provided? Large Tree b = 72’ Example 25 mph db a = 47’ 50 mph b d 62 50 mph ddbb = a __= a __ddaa____ ddaa -- bb da
- 32. 10/23/2012 32 63 da = 220 feet Large Tree b = 72’ Example 25 mph db a = 47’ 45 mph b d 64 45 mph ddaa = 220 feet= 220 feet ddbb = a __= a __ddaa__ =__ = 47’ (220’)47’ (220’) == 69.9’69.9’ ddaa –– b 220’b 220’ –– 72’72’ da
- 33. 10/23/2012 33 65 db = 69.9 feet corresponds to 15 mph Large Tree b = 72’ Example 25 mph db a = 47’ 45 mph b d 66 45 mph 25 mph > 15 mph, stopping sight25 mph > 15 mph, stopping sight distance is not sufficient fordistance is not sufficient for 25 mph25 mph da
- 34. 10/23/2012 34 Case B – Stop Control Three Sub Cases – Maneuvers • Turn left on to major roadway (clear traffic left enter• Turn left on to major roadway (clear traffic left, enter traffic right) • Turn right on to major roadway (enter traffic from left) • Crossing (clear traffic left/right) 67 Case B – Stop Control • Need ISD for departure and completion even if p p vehicle comes into view at point of departure = 1.47 Vmajor * tg where tg=7.5‐11.5s; add more for grade or multilane; decrease by 1s for right turns 68 or multilane; decrease by 1s. for right turns
- 36. 10/23/2012 36 Case C ‐ Yield Control • Minor Roadway Yields – must be able to see left/right – adjust speed – possibly stop • Sight distance exceeds that on stop control• Sight distance exceeds that on stop control • Similar to no‐control 71 Case C ‐ Yield Control • Must use minimum stopping sight distances for da and db rather than values from Table 7.7 (page 251, Garber and Hoel)(page 251, Garber and Hoel) • SSD calculation should include effect of grade • Required distance = P/R + stop 72
- 37. 10/23/2012 37 Case C ‐ Yield Control • Typically Known – a, b • Typically Assume Va or Vb 73 Case C ‐ Yield Control • Typically Known – a, b • Typically Assume Va or Vb • Similar triangle can be used to calculate safeSimilar triangle can be used to calculate safe approach speeds (given one approach speed) or allowable a and b. • da/db = (da – b)/a • db = (da *a)/ (da – b) 74
- 38. 10/23/2012 38 db 75 Critical speed is set to stopping distance dCritical speed is set to stopping distance dbb = (d= (daa *a)/ (d*a)/ (daa –– b)b) da Yield Control • Case C I: Crossing maneuver from minor road • Assumes that minor road vehicles that do not stop decelerate to 60% of minor road speed • Vehicle should be able to: • Travel from decision point to intersection decelerating to 60% of design speed C d l th i t ti t th d 76 • Cross and clear the intersection at the same speed
- 41. 10/23/2012 41 db da 81 ddbb = 82 ft to accommodate left and right turns= 82 ft to accommodate left and right turns da : similar to da for stopda : similar to da for stop--controlled but increasecontrolled but increase time gaps by 0.5 sectime gaps by 0.5 sec da: length of major approach 82
- 42. 10/23/2012 42 • Case D: Signal control First vehicle stopped should be visible to driver of other approaches 83 Also … • Case E: All way stop • Case F: Left turn from major • tg=5.5‐7.5s + multilane adjustmentg j • Effect of Skew 84
- 43. 10/23/2012 43 Sighting Rod and Target Rod (AASHTO) • For vertical sight distance with verticaldistance with vertical curves • Sighting rod‐ 3.5 feet tall • Target rod‐ 4.25 feet tall (Top portion and b f Sighting Rod 85 bottom 2 feet are painted orange) Target Rod Measuring at an Uncontrolled Intersection Assistant Obstruction X 86 Observer Decision Point Obstruction Y