1. INTERCHANGE DESIGN Fall 2017
Note: Some information taken from Freeway & Interchange: Geometric Design Handbook (2005) by J.P. Leisch et al. (ITE Publication)

2. Interchanges
Configuration governed by number of legs, expected volumes of through and turning movements, truck traffic and topography
Interchanges on freeways can significantly affect the community. Thus, they need to be located and designed so that they provide the best service possible
Once it is build, very expensive to modify or improve

3. Interchanges
Interchange: The connection of a freeway to a road or another freeway by a series of ramps (note: could be between two arterials). The other road or freeway being connected to is grade- separated.
Ramp: a roadway which connects a freeway to a regular road, a regular road to a freeway, or a freeway to a freeway
Directional Ramp: a ramp which turns directional to where it's going.

4. 3-leg Interchange

5. 4-leg Interchange

6. Warrants Design Designation Bottlenecks Safety Improvements
Whether the highway is full-access controlled or not
Need to decide how the cross-road will connect/intersect with the highway (frontage road, interchange, etc.)
Bottlenecks
Insufficient capacity for at-grade intersections
Note: right-of-way need to be available
Safety Improvements
Intersections that experience a disproportionate rate of severe crashes
The interchange will significantly reduce or eliminate these crashes

7. Warrants Site Topography Road-User Benefits Traffic Volume
At-grade intersections are infeasible because of terrain or too expansive to build
Road-User Benefits
Costs caused by delay, fuel usage, crash costs (where appropriate)
Need to evaluate time the vehicle will travel on the ramps as part of the benefit-cost analysis
Traffic Volume
Those will be governed by the capacity analysis and road-user benefits

8. 3-Leg: Trumpet Interchange

9. 3-Leg: Directional Interchange

10. 4-Leg: Diamond Interchange

11. 4-Leg: Cloverleaf Interchange

12. 4-Leg: Partial Cloverleaf Interchange

13. 4-Leg: Directional Interchange

14. Single-Point Interchange

15. Overpass versus UnderpassC L
Major Road
Crossroad
Underpass C L

16. Overpass versus Underpass
First consideration: Economic and maintenance costs (number, type and slope of ramps)
Undercrossing highways provide advance warnings to drivers that an exit is approaching
When turning traffic is important (exiting), the ramp profiles are better fitted when the major highway is at the lower level. The grade allows vehicle to decelerate more efficiently
When costs are not an issue, the selection between the two alternatives should be based on sight distance

17. Overpass versus Underpass
If there is a potential for drainage problem, then you should select an overpass
When a new highway crosses the major highway, building an overpass for the new highway causes less disturbances to the existing highway
There is no limitation for the vertical clearance for overpasses
Carrying the major highway under the crossing highway reduces the noise impact (underground)

18. Distance to attend Grade Separation

19. Distance to attend Grade Separation

20. Example
Compute the length needed to raise a major highway above the crossroad
Design speed 60 mph
Clearance: 16 ft
Structure: 8 ft

21. Example
The length needed is at least 1,200 ft, if use 3 percent grade

22. Distance to attend Grade Separation

23. Ex 3-76: ~ 450 ft (1/2 of 6% algebraic difference)

24. Lane Balancing at Interchanges
Basic number of lanes
Number of lanes on Ramp
The number of lanes beyond the merge area should not be less than the sum of all traffic lanes minus one
Example: 2 2 1

25. Lane Balancing at Interchanges
Basic number of lanes
Number of lanes on Ramp
The number of lanes prior the exit should not be less than the sum of traffic lanes after the exit on the main lane and the number of lanes in the exit minus one
Example: 4 3 2

26. Lane Balancing at Interchanges
Lanes are balanced, but problem with basic lane design 4 3 4 2 2
Basic lane design is adequate, but lanes are not balanced 4 4 4 2 2
Both basic lane design and lane balancing are adequate 4 5 4 5 4 2 2

27. Gore area / Speed-change lane
Components of a Ramp
Gore area / Speed-change lane
Ramp proper
Ramp terminal

28. Diagonal

29. Non-free-flow loop

30. Free-flow loop

31. Outer connection

34. Note: addressed in Part 2
Ramp Terminal
Distance long enough to provide for a speed-change lane
Normal intersection
Right turn by-pass
Note: addressed in Part 2

35. Gore Area - Exit Taper Design Parallel Design L L 12 ft 2o to 5o
250 ft min
(15:1 to 25:1)
L can be used on the curve only if the radius is larger than 1000 ft

37. Highway - Entrance Taper Design Parallel Design 12 ft L 16 ft L
300 ft min Lg Lg
Taper 50:1 to 70:1
Lg = 300 to 500 ft

40. Superelevation Design

41. Superelevation Design

42. Example Design a single-lane exit: Low traffic volume
Freeway: Design speed 70 mph, normal crown 1.5%
Ramp: Design speed 40 mph, radius = 900 ft, superelevation = 6.5%
Taper design

43. Example
Using Fig 3-36 (TxDOT RDM), the length available to decelerate should be 440 ft.
Taper angle of 2o = ~29:1.
The location at which the width is equal to 12 ft is 343 ft
12 ft
334 ft
440 ft

44. Example Compute the superelevation runoff for the exit ramp:
Lr = wn x ed x bw / Δ
Lr = 12 x 1 x (6.5 – 1.5) x 1 / 0.58 = 105 ft
1.5%
105 ft
6.5%