**DESIGNING THE HIGHWAY**

**INTRODUCTION**

PLANNING

Factors to be considered during planning

How will the proposed project affect the general physical character of the area surrounding the project ?

Coordination with Utilities

**Presented by**

P. Vipul Naidu

P. Vipul Naidu

**CONTENTS**

The entire length of expressway has a single layer of barbed wire fencing to keep out stray cattle.

**CONSISTENCY**

DEFINITION OF TERMS

THE DESIGN SPEED

CROSS SECTION OF TYPICAL HIGHWAYS

ROAD SHOULDER

THE CROSS SLOPE

CUT OR FILL SLOPE

NUMBER OF LANES

HIGHWAY MEDIAN

THE GRADE LINE

VERTICAL CURVE OVER CREST

RIGHT OF WAY

DEFINITION OF TERMS

THE DESIGN SPEED

CROSS SECTION OF TYPICAL HIGHWAYS

ROAD SHOULDER

THE CROSS SLOPE

CUT OR FILL SLOPE

NUMBER OF LANES

HIGHWAY MEDIAN

THE GRADE LINE

VERTICAL CURVE OVER CREST

RIGHT OF WAY

**STOPPING SIGHT DISTANCE**

THE PASSING SIGHT DISTANCE

ROAD ALIGNMENT

CIRCULAR CURVES

SUPER ELEVATION-RUNOFF

WIDENING OF CURVES

ISLAND

TYPES OF INTERCHANGE

HIGHWAY INTERSECTION AT GRADE

FREEWAY ENTRANCE AND EXIT

RAILROAD-HIGHWAY SEPARATION

BICYCLE LANE

THE PASSING SIGHT DISTANCE

ROAD ALIGNMENT

CIRCULAR CURVES

SUPER ELEVATION-RUNOFF

WIDENING OF CURVES

ISLAND

TYPES OF INTERCHANGE

HIGHWAY INTERSECTION AT GRADE

FREEWAY ENTRANCE AND EXIT

RAILROAD-HIGHWAY SEPARATION

BICYCLE LANE

CONSISTENCY

-the most important single rule in highway design

Drivers expect the highway agency to provide them with:

DEFINITION OF TERMS

ROADS AND HIGHWAYS

ROAD

HIGHWAY

EXPRESSWAY

FREEWAY

CONTROL OF ACCESS

FULL CONTROL OF ACCESS

PARTIAL CONTROL OF ACCESS

THROUGH STREET OR THROUGH HIGHWAY

PARKWAY

ARTERIAL STREET

COLLECTOR STREET

LOCAL ROAD

HIGHWAY CAPACITY

DESIGN SPEED

CROSS SECTION of TYPICAL HIGHWAY

variables to consider such as:

(1) the volume of traffic

(2) character of the traffic

(3) speed of the traffic

(4) characteristics of motor vehicles and of the driver

A cross section design generally offers the expected level of service for safety and a recent study shows that:

A 7.20 meters wide pavement has 18% less accident compared with pavement narrower than 5.50 m wide.

A 7.20 meters wide pavement has 4% fewer accidents than the 6.60 m wide roadway.

Accidents records showed no difference between 6.60 meters and the 7.20 m wide pavement.

For the 6.00 m,6.60 m, and 7.20 m wide pavement with 2.70 to 3.00 m wide shoulder, recorded accident decrease by 30% compared to 0 to .60 m wide shoulder. And 20% compared with a .90 to 1.20 m wide shoulder.

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cross section for:

two lane rural highways

collector roadway

local rural highway

urban roadway

Importance of road shoulder

Policy on Geometric design

Road shoulder

Road Shoulder

Importance of Road Shoulder

Policy on Geometric Design Recommendation

Vertical Curve Over Crust

" The vertical distance fro the intersection of the straight grade line to the curve is equal to the curve is equal to one eight of the product of the algebraic difference in grades andthe length of the curve in stations."

Right of Way

Acquisition of land for the right of way is very costly.

Stopping Sight Distance

It is defined as the longest distance that a driver could see the top of an object 15 cm above the road surface where the design height of the driver's eye above the pavement is 105 cm.

d=------------

(m/s)

19.6f

2

d= ----------

(m/s)

19.6(f+g)

2

flat surface:

uphill or downhill:

The Passing Sight Distance

It is defined as the longest distance that a driver can see the top of an oncoming vehicle where the driver's eye level is 1.05m above the pavement surface.

L = 2S - 943

______

A

L = 2AS

943

______

when S>L

when S<L

ROAD ALIGNMENT

- should be consistent

- abrupt change from flat to sharp curve should be avoided

- designing circular curves of different radius is not a good practice

- having a short tangent between two curves is a poor practice

HIGHWAY INTERSECTION AT GRADE

1. For right angle intersection with little traffic, use street signs.

2. For Y intersection or other related conformation, requires channelization.

3. For flared design will involve:

a. Widening the entering traffic lane

b. Widening the leaving lanes

4. Must accomodate large trucks.

Freeway Entrance and Exit

Overall effectiveness is governed on:

1. Flow characteristics of vehicles

2. Driver's behavior near on and off the channel

Single lane on ramp vehicle:

- design is either taper blending

- sufficient length is required

Railroad - highway Separation

- requires minimum of 7 meters as against 4.8 meters clear distance above the highway

- requires special provision when highways are under railroads

Bicycle Lane

- design speed is 20 to 30 km/hr

- width is 2 meters (minimum) for two way travel

- grade is 5% (maximum) on short distances

CIRCULAR CURVES

The sharpness of a curve is dependent on its radius.Sharpness is expressed in terms of the curve degrees, and the degrees of curve is inversely proportional with the radius.

D= 5729.58

___________

RADIUS

Arc Definitions-the degree of curve is the central angle subtended by a 30 m arc of the curve

Chord Definitions-the degree of curve is the central angle subtended by 50 m chord

Radius= 50

__________

sin (1/2) D

Super Elevation-Runoff

Widening of Curves

A provision for a wider roadway is necessary on a sharp curve for two lanes pavement under the ff. reasons:

1.) Two force drivers to shy away from the pavement edge.

2.) To increase the effective transver se vehicle width for non-tracking of front and rear wheels.

3) To give additional width due to the slanted position of the front wheel to the roadway center line.

4) For a 7.20 meters wide roadway, an additional width of 30 cm is necessary on an open curve highway.

ISLAND

An island is a defined area between traffic lanes for control of vehicle movement and for pedestrian refuge.

It is included in the design of intersections for the following purposes:

Separation of vehicular flows.

Separation of conflicts.

Reduction in excessive pavement areas.

Reduction of traffic and indications of proper use of intersections.

Arrangement to favor a prominent turning movement.

Location of traffic control devices.

TYPES OF INTERCHANGE

To facilitate the easy transfer of vehicles from one entry to the other or between local roadway and the freeway.

The types and form of freeway interchange requires selection of the conformation that is best suited to a particular situation and demand. The functions of freeway interchanges are:

To provide separation between two or more traffic arteries.

Objections to the Cloverleaf Interchange Design

The Cross Slope

a cross slope in one direction of multi lane highways makes driving comfortable, with heavy rainfall, the water depth increases on the highway.

Cut or Fill Slope

Earth fill of normal height is safe on the slope of 1:2 ratio.

Advantages of Flat Side or Back Slope

With back slope of 3:1 o even flatter, cars could be directed to back into the road and will come to stop or continue down the slope with no risk of over turning.

Flat fill slopes are visible from the vehicles at full extent giving the roadway safer appearance.

With visible slope for being low and flat, vehicles could be positioned closer to the edge, and on two lane roadway facilities parking would be farther from the opposing traffic.

Recommended Policy on Geometric Design

The 6:1 Slope ratio could be adopted on embankment less than 1.20 meters high, and 4:1 ratio on a higher fill.

The 2:1 slope is allowed to heights greater than 6.00 meters.

Cut slope should not be steeper than 2:1 ratio except on solid rock or special kind of soil.

AASHTO-

Stipulated that where cut or fill slopes intersects the original ground surface, cross section must be rounded to blend the slope with the natural ground surface.

Materials could be used when the slide slope requires embankment:

Hand placed stones

Cement rubbles masonry

Concrete blocks

Conventional reinforced concrete

T or counter forted designs

Cribs assembled from timber

Precast concrete

Metal elements

Number of Lanes

The number of lanes in the segment of the highway is determined from the estimated traffic volume for the design year (AADT) and highway lane capacity at expected level of service.

Highway Median

median in various forms, becomes absolute requirement for highways because it offers many advantages.

the ff.:

For narrow median, there are four means of reducing cross median accidents.

Provide deterring devices

Provide non-traversable energy absorbing barriers

Provide non-traversable rigid barriers.

Provide G.M. barriers

In flat area, the grade line is set almost parallel to the ground surface but sufficiently above the ground for drainage purposes.

Undesirable native soil should be provided with sufficient covering.

Grade line elevations along the river or stream, is governed by the expected level of water flood.

In laying the grade line, the designer must consider the following:

Where earthwork is minimal and consistently meeting sight distances in relation to grade line, economy is one main consideration

In mountainous area, the grade line must be considered balanced excavation against embankments to get the minimum overall cost

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