Title: Chapter 6 Design of Highway At-Grade Intersections
1Chapter 6Design of Highway At-Grade Intersections
- typical intersection types and their
characteristics - general design principles for at-grade
intersections - Know how to find minimum radii for the curves at
at-grade intersections - Know what the channelization is, objectives for
using channelization, and factors affecting its
application
2Intersection types
At-grade
Grade separated
With ramps (Interchanges)
Without ramps (meaning no connection between the
intersection roads!)
3Topics to be covered
- Types of intersections
- Objectives in the design of at-grade
intersections - Alignment considerations
- Grade considerations
- Curve types and minimum radii
- Channelization what it is, why we need it, how
we provide it
4At-grade intersections (pay attention to
channelization techniques) T or 3-leg
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7Speed change lanes
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9At-grade intersections (pay attention to
channelization techniques) 4-leg
10At-grade intersections (pay attention to
channelization techniques) Multi-leg
Mulry Square in Greenwich Village, Manhattan, New
York City
11- Rotary Intersections (Roundabouts)
- A rotary intersection is one in which all traffic
merges into and emerges from a one- way road
around a central island. There is little delay to
traffic due to speed reductions and no delay at
all due to stopping. Its advantages include - 1. Continuos traffic movement from all legs
- 2. Accidents are likely to be less serious
- 3. Where more than 4-legs are involved , the
design layout may be simplified - 4. The cost of this type may be considerably less
than that of grade separation structures.
12- Disadvantage include
- 1. Requires large area
- 2. Costs are more than other at-grade
intersections - 3. It is not suitable for large pedestrians
movements - 4. There is little delay due to speed reduction
- 5. It needs long weaving sections to ensure
smooth flow.
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14Typical markings for roundabouts with one lane
Roundabouts different from circles
- Circles may have a signal, STOP signs,
no-control at entry Roundabouts are always
controlled by Yield signs. - Splitter islands
- Peds are not allowed to use the central island
- No parking in the circle
- Circulating vehicles always have the ROW.
15Roundabouts different from circles
- Advantages
- Provides non-stop movements
- Reduce crash occurrences
- Reduce crash severity
- Esthetically appealing
- Function as a traffic calming measure
In Australia
Good for low to medium traffic. Definitely NOT
for high volume intersections ? Too many weavings
In Maryland
In Norway
16At-grade intersection design objectives and
considerations
- Need to meet two conflicting objectives
- Minimize the severity of potential conflicts
among different streams of traffic and between
pedestrians and turning vehicles. - Provide for the smooth flow of traffic across
the intersection
Adequate pavement width and approach sight
distances must be provided.
Operating characteristics of both the vehicles
and pedestrians
17At-grade intersection design considerations
- Alignment and profile design
- Angle of intersecting roads
- Suitable channelization system for the traffic
pattern
- Minimum required widths of turning roadways
- Adequate sight distance for the type of traffic
control used (no control, Yield, Stop, Signal)
18Alignment
Suggested improvements to intersections with
acute angle problems
Longer walking distance
Angle of turn
Obtuse angle
The angle of turns should be 60 to 120 degrees.
Acute angle
Superelevation problem
Wider pavement needed for turning vehicles
19Profile
- Make it as flat as possible
- Avoid approach grades in excess of 3
- Avoid grade changes at intersections (Crest ?
sight distance problems, Sag ? drainage problems) - The grade line of the major highway should be
carried through the intersection. Adjust the
grade for the normal crown of the crossroad to an
inclined cross section at its junction with the
major road. ? A good example at Columbia Lane and
Grandview Road (the signalized intersection on
Columbia Lane just north of DI).
20Curves at at-grade intersections
Rule 1 When the turning speed at an intersection
is assumed to be 15 mph or less, the curves for
the pavement edges are designed to conform to at
least the minimum turning path of the design
vehicle. If the speed is greater than this, the
design speed is also considered to determine the
radius (Remember? R u2/(ge fs). ? This
means that you are not supposed to use the values
in Table 7-2 and 7-3 in the textbook. Rule 2 The
angle of intersection affects the curve design.
- Typical types
- Simple curve
- Simple curve with taper
- 3-centered compound curve
21Three typical curve design methods
22Minimum edge of pavement designs Simple curve
and simple curve with taper (Table 7.2)
R
23Channelization objectives
- Direct the paths of vehicles
- Control the merging, diverging, and crossing
angle of vehicles - Reduce the amount of paved area
- Provide a clear indication of the proper path
for different movements - Give priority to the predominant movements
- Provide pedestrian refuge
- Provide separate storage lanes for turning
vehicles
- Provide space for traffic control devices for
visibility - Control prohibited turns
- Separate different traffic movements at
signalized intersections with multiple-phase
signals - Restrict the speeds of vehicles
24Channelization considerations
- Motorists should not be required to make more
than one decision at a time. - Sharp reverse curves and turning paths greater
than 90 degrees should be avoided. - Merging and weaving areas should be as long as
possible, but other areas of conflict between
vehicles should be reduced to a minimum - Crossing traffic streams that do not weave or
merge should intersect at 90 degrees, although a
range of 60-120 degrees is acceptable
- The intersecting angle of merging streams should
be such that adequate sight distance is provided. - Refuge areas for turning vehicles should not
interfere with the movement of through vehicles - Prohibited turns should be blocked wherever
possible - Decisions on the location of essential traffic
control devices should be a component of the
design process.
25Chapter 7Grade Separations- and interchanges
Intersections
- Although the construction cost of the grade
separated structures is very high, but the
following advantages justify its uses - 1. To ensure free movement of high speed traffic
on motorways - 2. Increase capacity lane to its initial value
- 3. Increase safety for all vehicles
26- 4. More economic in operation and losses and
cheaper in expensive areas - 5. Save time and money and more comfort
- 6. It can be implemented at any site.
27- The separated at-grade intersections have the
following disadvantages - 1. High construction costs and maintenance
- 2. Not suitable for five or more legs
intersections - 3.The layout of grade separation may be confusing
to some drivers .
28- Interchanges can be divided into the following
main types - a) 3- way interchanges may be a T -or Y types
intersection as shown in Fig.34. Both types
utilize a single bridge structure and are
suitable for expressway provided that the loop
movement is relatively small. In the case of
heavy movement, an extra bridge may be
constructed so that both turning movements are
favored equally.
29- b) 4- way interchanges the simplest type of
4-way interchanges is the diamond, which
consisting of a single bridge and four way ramps.
This type can be located within relatively narrow
land area. It has high speed entrance and exit
ramps on the main road and at -grade ramp
terminals on the minor road.
30- c) The Cloverleaf It is only 4- way single
structure interchange having no terminal left
turns at-grade. It is the most common form of
interchange and is required as the ultimate
answer to intersection problems. It being very
uncomplicated to use with
31Examples of grade separated interchanges
32Examples of grade separated interchanges (cont)
33More examples of directional interchanges
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