Intersection Design• Objectives: facilitate the

safety, efficiency, convenience, ease, and comfort of vehicular, cyclist, and pedestrian movements at an intersection.

• Means: movement channelization, approach re-alignment, curb return design, sight distance, signal or stop control,

• Design criteria: human factors, traffic characteristics, land use and environment, economic considerations

Definitions• Intersection: general area

where two or more highways join or cross.

• Intersection leg: a highway radiating to/from an intersection.

• Intersection types: at-grade, grade separations without ramps, and interchanges.

• At-grade intersection types: T or three-leg intersections, four-leg or cross intersections, multileg intersections.

• Channelization: the provision of facilities such as pavement markings and traffic islands to regulate and direct conflicting traffic streams into specific travel paths.

Conflicts at at-grade Intersections

ChannelizationChannelization• Exhibit A, next page is the most

common type of T intersection.• Paved width is maintained except for

the paved returns and where widening is needed to accommodate design vehicle.

• Exhibit A design is suitable for 1) junctions of minor or local roads and 2) junctions of minor roads with more important highways where the intersection angle does not deviate from perpendicular by more than 30 degrees.

• Exhibit A design is very common in rural areas in conjunction with two-lane highways with light traffic.

• As speed and turning movements increase, an additional area of surfacing or flaring may be provided.

• Auxiliary left and right-turn lanes increase capacity and create better operational conditions for turning vehicles as in the next three exhibits.

Channelization

Channelization

Channelization• Exhibit A (next page) displays a single

turning roadway realized by increasing the return radius between the two roadways.

• Return radius in increased sufficiently to permit a turning road, which is separated from the normal traveled way of the intersecting approaches by an island.

• Exhibit B shows a pair of turning roadway.

• Exhibit B design is suitable where above minimum speed or turning paths are to be provided.

• Following two exhibits provide for the flaring of the through highway to accommodate left turns from the main highway.

• When separate turning roadways are necessary, flaring is usually necessary as well.

• The first of the latter exhibits depicts a divisional island on the intercepted road.

• Space for this island is created by flaring the pavement edges of the intercepted road and by using larger than minimum pavement edge radii for right turns.

• The second shows a divisional island with two right turning roadways on the intercepted road. This configuration is desirable where intermediate to heavy traffic volumes are carried.

Channelization

Channelization

Channelization

Channelization

Channelization

Channelization

Channelization

Channelization

RealignmentRealignment• Intersections should cross at or nearly

at right angle for safety and economy. • Roads intersecting at acute angles

need extensive turning roadways.• Truck drivers turning at acute angles

have blind areas on the right side of the vehicle.

• Acute angle intersections increase the exposure of the crossing traffic to the main street.

• Realigning roads intersecting at an acute angle in the manner shown on the next page has proven beneficial.

• Methods A and B, which create short radii curves on the side road to achieve right-angle intersections should be avoided.

– Such methods result in increased lane encroachments.

– Also, traffic control devices may be out of the driver’s line of sight resulting in advanced signage.

• Offset intersections are preferable as displayed in methods C and D.

– Method C forces a left turn onto the major road by minor road vehicles and is appropriate where minor road traffic is light. (This methods introduces added left turns on the main road.)

– Method D is preferable as a right turn on the main road is executed by minor road vehicles.

– Method E is desirable where a minor road is located along a tangent to a main road that curves.

Realignment

Realignment (to minor road)

Edge of pavement curves (Curb return)

• Given turning speeds of 15 mph or less, curves for the pavement edge are designed to conform to at least the minimum turning path of the design vehicles.

• When the turning speed is expected to be greater than 15 mph, the design speed is also considered.

• Curb return types:– Simple curve– Simple curve with taper– 3-centered compound

curve

Curb return

Curb return

Curb return

Curb return

Curb return

Intersection Sight Distance• The provision of stopping sight distance at

all locations along highways, including intersections, is fundamental to intersection operation

Intersection Sight DistanceDefinitions

• Clear sight triangle: Specified areas along intersection approach legs and their included corners cleared of obstructions that might block a driver’s view of potentially conflicting vehicles.

• Approach sight triangle: Clear sight triangle that enables an approaching vehicle driver on the minor road to view potentially conflicting vehicles.

• Departure sight triangle: Clear sight triangle that enables a stopped vehicle on a minor road to depart from the intersection.

Intersection Sight Distance

Sight Distance versus Control

Case A – No Control

Sight Triangle Leg

Case B

Case B1

Case B1 – Time Gap

Case B1 – Sight Triangle Leg

Cases B2 and B3 – Time Gap

Cases B2 and B3 – Sight Triangle Leg

Case C1

Case C1 – Time Lag

Case C1 – Sight Triangle Leg

Case C2 – Time Gap

Case C2 – Sight Triangle Leg

Intersection Sight DistanceYield control – Stop on minor

Intersection Sight DistanceYield control

Intersection Sight DistanceStop Control

Intersection Sight Distance – Stop Control (Case IIIa – Crossing Maneuver)

Intersection Sight Distance – Stop Control (Case IIIa – Crossing Maneuver)

Intersection Sight Distance – Stop Control (Case IIIb – Left Turning Maneuver)

Intersection Sight Distance – Stop Control (Case IIIb – Left Turning Maneuver)

Intersection Sight Distance – Stop Control (Case IIIb – Left Turning Maneuver)

Intersection Sight Distance

Intersection Sight Distance – Stop Control (Case IIIc – Right Turning Maneuver)

Layout of Concentric Curves

• Same deflection angles on all three curves

• Different chord lengths

Earthwork• Definition: All excavations and fills (for

structures embankments, borrow, overhaul, machine grading, subgrade scarifying, rock fill, reconditioning roadway) to prepare the subgrade foundation for highway or runway pavement or roadway ballast.

• Measurement: Cubic Yard of Excavation, square yard (shallow excavation)

Earthwork• Classification of Excavated Material

– Common excavation– Loose rock– Solid rock

• Shrinkage and Swell Factors– Excavated earth when placed in fills shrinks

by 10 – 40 % (becomes compacted and occupies less volume)

– Excavated rock swells (5 – 10 %)

Earthwork

Cross Section Area

• Graphical or Planimeter Method• Coordinate Method

Cross Section Area

Cross Section Area

Cross Section Area

Earthwork Volume

• Average End Area Formula

• Prismoidal Formula

Mass Diagram• Definition: A continuous curve showing the

cumulative sum of yardage from the original station to any subsequent station.

Earthwork

Earthwork