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

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

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.

3-leg Interchange

4-leg Interchange http://www.angelfire.com/on3/DonovanMartin/interchanges.html

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

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

3-Leg: Trumpet Interchange

3-Leg: Directional Interchange

4-Leg: Diamond Interchange http://www.angelfire.com/on3/DonovanMartin/interchanges.html

4-Leg: Cloverleaf Interchange

4-Leg: Partial Cloverleaf Interchange

4-Leg: Directional Interchange

Single-Point Interchange http://www.angelfire.com/on3/DonovanMartin/interchanges.html

Overpass versus Underpass C L Major Road Crossroad Underpass C L

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

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)

Distance to attend Grade Separation

Distance to attend Grade Separation

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

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

Distance to attend Grade Separation

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

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

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

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

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

Diagonal

Non-free-flow loop

Free-flow loop

Outer connection

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

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

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

Superelevation Design

Superelevation Design

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

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

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%