interchange design

Interchange design

Raghupathi

Outline

1. Introduction & Need

2. Types of Interchanges

3. Components of interchange

4. Interchange design considerations

5. Design specifications

6. Lane balancing & weaving area

7. Ramp design

Introduction

An Interchange is a system of interconnecting roadways in conjunction

with one or more grade separations that provides for the movement of

traffic between two or more roadways or highways on different levels.

Need

• Freeways are fully access controlled arterials

• Purpose: move large volumes of traffic at high speed, safely and

efficiently

• At-grade crossing are

prohibited : Will make

the vehicles stop.

Interchange Warrants

1. Design Designation

2. Congestion

3. Safety

4. Topography

5. Traffic Volume

6. Road-User Benefits

Greatest efficiency,

safety and capacity

are attained

Types of interchanges

1. Diamond

2. Cloverleaf

3. Directional

1. Diamond

• Use where intersection can handle left turns

• Simplest

• One-way diagonal ramp in each quadrant

for all turning movements

• Ramp originates/terminates at either

at-grade intersection with cross street

or junction with frontage roadSource: A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. Chapter 10 Grade Separations and Interchanges

Overpass or underpass?

• Depends on topography, economy, and other

minor factors

• Underpass better for deceleration/acceleration,

cost, and advance warning

• Overpass better for aesthetics

2. Cloverleaf

• Full and partial

• Apply where left turns can’t be

served (possibly physically) at an

intersection and there is the

available ROW

• Loops accommodate LTs

Advantages of Diamond Over cloverleaf

• Requires less RW

• Left-turns travel less distance

• High speed exit/entrance

• Has expected exit/enter pattern

(versus cloverleaf with

unexpected pattern and weave

section)

• No weaving section

Advantages of Cloverleaf over Diamond

• Left turns merge at acute angles

• Higher left turn capacity

• Turning vehicles may not have to

stop

• Don’t need median/signs to

prevent wrong way entrance

3. Single Point Urban Interchange

A Single Point Urban Interchange (SPUI)

is controlled by one set of traffic signals

(highlighted) located at a single point at

the center of the interchange. The signals

direct thru-traffic, as well as all traffic that

must turn left to enter or leave the

freeway

Advantages of SPUI

• All 4 LT movements controlled

by single traffic signal

• RT are free flow movements

• Good with narrow ROW

• Higher capacity

Disadvantages of SPUI

• High construction costs

• Difficult for pedestrians and bicyclists to negotiate

• Complex intersection and signal phases may be unfamiliar to drivers

• Streets can lead to very large areas of uncontrolled pavement

• Vehicle clearance time is longer

• Require larger bridge girder

• More free-flow motor vehicle movements

4. Directional/Semi-Directional

• Apply: freeway to freeway or other intersecting roadways with major

turning movements (where diamond, cloverleaf or SPUI can’t handle

the volumes)

• Allows higher LOS

• Advantages over cloverleaf

• Higher speed left turn

• less weaving (often none)

• normal exit/entrance patterns

Characteristics/Comparison of Basic Interchange Types

Different Configuration of interchanges

Source: A Policy on Geometric Design of Highways and Streets, AASHTO, 2004. Chapter 10 Grade Separations and Interchanges

Ramp Types

• Diagonal

• One-quadrant ramp

• Semi directional

• Outer connection

• Directional

• Loops


Interchange components

https://www.fhwa.dot.gov/publications

Factors effecting to select the type of interchange are

1. Highway classification

2. Character and composition of traffic

3. Design speed

4. Degree of access control

Interchange Design Considerations

1. Determination of Interchange Configuration

• Interchange configurations are covered in two categories,

System interchanges: interchanges that connect two or more freeways

Service interchanges: interchanges that connect a freeway to lesser facilities

• Different alternates are prepared based on the land use and are

compared to figure out the best

Standard: System (freeway to freeway – directional)

Service (freeway to arterial or collector – diamond, cloverleaf, etc.)

Principles to compare different alternatives

1. Capacity

2. Route continuity

3. Uniformity of exit patterns

4. Single exits in advance of the separation structure

5. With or without weaving

6. Potential for signing

7. Cost

8. Availability of right-of-way

9. Constructability

10. compatibility with the environment.

2. Alignment, Profile and Cross Section

• The design speed, alignment, profile and cross section in the interchange area

should be consistent with those on the approaching highways.

• Four-lane roadways should be divided at interchanges

• At-grade left turns preferably should be accommodated within a suitably wide

median

3. Sight Distance

• Sight distance on the roadways through an interchange should be at a minimum

the required stopping sight distance preferably “Decision Sight Distance”

4. Interchange Spacing

• Minimum spacing is determined by

• Weaving requirements

• Ability to sign

• Lengths of speed change lanes

• Capacity of the main facility

• Interchanges an average minimum spacing of

• Urban areas: 2 miles

• Suburban sections 4 miles

• Rural areas 8 miles.

• In urban areas, the minimum distance between adjacent interchanges

should not be less than 1 mile, and in rural areas not less than 3 miles

5. Uniformity of Interchange Patterns

• Interchanges along a freeway should be reasonably uniform in geometric layout

and general appearance.

• Provides the appropriate LOS and maximum safety in conjunction.

• Highly special cases, all entrance and exit ramps should be on the right.

6. Route Continuity

• Simplifies the driving task

Reduces lane changes

Simplifies signing

Delineates the through route

7. Signing and Marking

• Safety, efficiency and clarity of paths largely depends on Signing and Marking

• Signing and marking should conform to the OMUTCD (Ohio Manual of Uniform Traffic

Control Devices)

8. Basic Number of Lanes

• The minimum number of lanes needed over a significant length of a highway

based on the overall capacity needs of that section.

• The number of lanes should remain constant over short distances

Interchanges on Freeways as Related to Types of Intersecting Facilities and Surrounding Area are shown


Design specifications

1. Cross slope and shoulder

• Minimum 2 through lanes 1.5 to 2% cross-slope

• Continuous paved shoulder

Right: 10 ft

Left: 4 to 8 ft

2. Clearance

• Vertical clearance: at least 16 ft over entire cross-section

(Consider future resurfacing)

• Horizontal: clear zone consistent with operating speed and side slopes

Medians: Rural: 50 to 100 ftUrban

For 4 lane use 10 ft (2x4 ft shoulder + barrier)

For 6 lane use 22 ft (2x10 ft shoulder + barrier)

3. Minimum Grades for urban and rural freeways


Minimum distance required to attain effect grade separation


4. Design speed:

5. Design Widths for Turning Roadways


Lane balance

• Number of mainline lanes leaving the diverging

nose must be equal to the number of mainline

lanes approaching the nose.

• The total number of lanes leaving the diverging

nose (mainline lanes plus diverging lanes)

must be one greater than the total number

of lanes approaching the nose to obtain

lane balance

http://onlinepubs.trb.org


Typical example of lane balance Coordination of lane balance and basic number of lanes

Multilane exit ramps and diverging roadways


High speed two-lane exit terminals


Weaving Areas

• Weaving occurs where one-way traffic streams cross by merging and diverging maneuvers


Weaving Configuration


Major Forks and Branch Connections

• Major forks are where a freeway separates into two distinct freeways.


Ramp Design

Ramp: All types, arrangements, and sizes of turning roadways that

connect two or more legs at an interchange

1. Geometric design

It includes

1. Design speed

2. Cross section

3. Horizontal alignment

4. Vertical alignment

1. Design speed

2. Cross Section

• Ramp Width –22 feet for one-lane ramps and 30 feet for two-lane ramps.

• Cross Slope – Tangent sections-uniformly sloped at 2.0% from the median edge to

the opposite edge. maximum super elevation rate at 6.0%.

• Side Slopes – should be 1:6 or flatter.

• Lateral Clearances to Obstructions (Clear Zones)

o Clear zone widths vary from 6-10 feet at 40 mph to 40-50 feet at 70 mph.

o lateral clearance on the right outside of the edge of traveled way of at least 6 feet and

preferably 8 to 10 feet,

o lateral clearance on the left of at least 4 feet beyond the edge of the traveled way

• Exit Ramp Entrance Width

o Where the through lane and exit ramp diverge, width will be 25 feet

o This width will be maintained until the gore nose is reached and transitioned to the standard 22

feet width at approximately a 12:1 rate.

• Entrance Ramp Terminal Width

o The standard 22 feet width will be transitioned to 14 feet width at the convergence with the

through lane

Typical Sections for Ramps


3. Horizontal Alignment

Depends on

• Design Speed: As shown above

• Outer Connection : At cloverleaf interchanges should be as directional as possible

• Loops: Continuously curved alignment in a compound curve arrangement

• Super elevation: Maximum super elevation rate at 6.0%

• Sight Distance: As great as the design stopping sight distance

• Two-Lane Ramps: The desirable minimum radius is 1,000 feet

Two-Lane Exit Terminals

4. Vertical alignment

• Minimum grade: 0.50%

• Maximum grade: Cannot be as definitively expressed as for highway mainline


2. Capacity

• Although up to 1,700 pcph can be accommodated on a single-lane ramp,

freeway/ramp junctions are not capable of handling this volume

• 1,500 pcph should be used as a threshold.

interchange design

Engineering