Highway Engineering

Highway Engineering

Geometric Design of Highways

INTRODUCTIONThe design of highways necessitates the determination of specific design elements, which include the number of lanes, lane width, curve radii required for vehicle turning, and alignment required to provide appropriate stopping and passing sight distances.

OBJECTIVEto maximize the comfort,

safety,

minimizing cost and

environmental damage

PRINCIPLES OF HIGHWAY ALIGNMENTHighway Alignment is a three-dimensional problem.Design & Construction would be difficult in 3-D so highway alignment is split into two 2-D problems

COMPONENTS OF HIGHWAY ALIGNMENTHorizontal Alignment

Vertical AlignmentPlan ViewProfile View

HORIZONTAL ALIGNMENTis the route of the road, defined as a series of horizontal tangents and curves.The design of a horizontal curve entails the determination of a minimum radius (based on speed limit), curve length, and objects obstructing the view of the driver.If a horizontal curve has a high speed and a small radius, an increased superelevation (bank) is needed in order to assure safety. If there is an object obstructing the view around a corner or curve, the engineer must work to ensure that drivers can see far enough to stop to avoid an accident or accelerate to join traffic.

TangentsCurves

Tangents & Curves

TangentCurve

Tangent to Circular CurveTangent to Spiral Curve toCircular Curve

8

Types of Horizontal AlignmentsSimple or Circular CurvesCompound CurvesReverse CurvesSpiral Curves

1. Simple CurveThe simple curve is an arc of a circle. It is the most commonly used. The radius of the circle determines the sharpness or flatness of the curve. The larger the radius, the flatterthe curve.

Simple Curve

2. Compound CurveCombination of two or more simple circular curves of different radius having their curvature in the same direction. Essentially, a compound curve consists of two curves that are joined at a point of tangency and are located on the same side of a common tangent. Though their radii are in the same direction, they are of different values.

Compound Curve

3. Reverse CurveA reverse curve is a civil engineering concept pertaining to two connected simple curves that share a common tangent. Such structures are used in channelized intersections, interchange ramps and median crossovers. They are also sometimes used to realign crossroads.

Reverse Curve

4. Spiral CurveThe spiral is a curve with varying radius used on railroads and somemodern highways. It provides a transition from the tangent to a simple curve or between simple curves in a compound curve.

Spiral Curve

Superelevation

Superelevation

19Divide both sides by Wcos()

Assume fse is small and can be neglected it is the normal component of centripetal acceleration

Vertical Alignmentis the vertical aspect of the road, including crest and sag curves, and the straight grade lines connecting them.The profile also affects road drainage. Very flat roads and sag curves may have poor drainage, and steep roads have high velocity flows.

The vertical alignment of a transportation facility consists of tangent grades (straight line in the vertical plane) vertical curves. Vertical alignment is documented by the profile.

Types of Vertical Alignment 1. Crest Vertical CurvesSymmetrical Unsymmetrical

2. Sag Curves

Crest Vertical CurvesCrest vertical curves are curves that connect inclined sections of roadway, forming a crest, and they are relatively easy to design.As you know from the module entitled Vertical Curves, we only need to find an appropriate length for the curve that will accommodate the correct sight distance.The stopping sight distance is usually the controlling sight distance, but the decision sight distance or even the passing sight distance could be used if desired.

Crest Vertical Curves

G1G2PVIPVTPVC

h2h1

L

SSD

For SSD < LFor SSD > LLine of Sight

Design Controls for Crest Vertical Curves

Sag CurvesSag vertical curves are curves that connect descending grades, forming a bowl or a sag. Designing them is is very similar to the design of crest vertical curves. Sag vertical curves are characterized by a positive change in grade, which means that vehicles traveling over sag vertical curves are accelerated upward.Because of the inertia of the driver's body, this upward acceleration feels like a downward thrust.

Sag Vertical Curves

G1G2PVIPVTPVC

h2=0h1

L

Light Beam Distance (SSD)

For SSD < LFor SSD > L

headlight beam (diverging from LOS by degrees)

Design Controls for Sag Vertical Curves

Stopping Sight Distance

Rvs

ObstructionMs

SSD

33Basically its figuring out L and M from the normal equations

VERTICAL ALIGNMENTis the vertical aspect of the road, including crest and sag curves, and the straight grade lines connecting them.The profile also affects road drainage. Very flat roads and sag curves may have poor drainage, and steep roads have high velocity flows.

CREST CURVECrest vertical curves are curves which, when viewed from the side, are convex upwards.

The most important design criterion for these curves is stopping sight distance. This is the distance a driver can see over the crest of the curve. If the driver cannot see any obstructions in the roadway, such as a stalled vehicle or animal, the driver may not be able to stop the vehicle in time to avoid a crash.

SAG CURVESag vertical curves are curves which, when viewed from the side, are concave upwards.

The most important design criterion for these curves is headlight sight distance. When a driver is driving on a sag curve at night, the sight distance is limited by the higher grade in front of the vehicle. This distance must be long enough that the driver can see any obstruction on the road and stop the vehicle within the headlight sight distance.