Lec 04 Highway Engineering - Curve Widening

Lecture 04 40

Highway Eng. Widening on Horizontal Curves 14 15

Dr. Firas Asad

In this lecture; ---------------------

A- Definition and justifications.

B- Formula & calculations.

C- Application of Widening.

Travelled Way Widening on Horizontal Curves

The information listed in this lecture is mainly taken from the Policy on Geometric Design of Highways and Streets (AASHTO, 2011), Iraqi Highway Design Manual (SORB, 2005) and Traffic and Highway Engineering (Garber and Hoel, 2009).

Travelled Way Widening on Horizontal Curves

A- Definition and Justifications.

The travelled way on horizontal curves is sometimes widened to make operating

conditions on curves similar to those on tangents. The added width occupied by the

vehicle as it traverses the curve as compared with the width of the travelled way on

tangent can be computed by geometry for any combination of radius and

wheelbase.

Widening is needed

on certain curves for one of the following reasons: (1) the

design vehicle (especially larger ones) occupies a greater width because the rear

wheels do not follow precisely the same path as the front wheels (offtracking) when

the vehicle traverses a horizontal curve or makes a turn, or (2) drivers experience

difficulty in steering their vehicles in the centre of the lane.

Lecture 04 41


Dr. Firas Asad

-- w = Wc - Wn

w = widening of travelled way on curve, m; Wc = width of travelled way on curve, m; Wn = width of travelled way on tangent, m

The travelled way width

needed on a curve (Wc) has

several components related to

operation on curves (see the

adjacent figure).

B- Widening Formula & Calculations

The amount of widening w of the travelled way on a horizontal curve is the

difference between the width needed on the curve Wc and the width used on a

tangent Wn:

Lecture 04 42


Dr. Firas Asad

--Wc = N(U + C) + (N - 1) FA + Z

where: N = number of lanes (per direction for divided Hwy.); U = track width of design vehicle (out-to-out tires), m; C = lateral clearance, m; FA = width of front overhang of inner-lane vehicle, m; Z = extra width allowance, m

U = u + R - U = track width on curve, m; u = track width on tangent (out to-out of tires), m; R = radius of curve or turn, m; and Li = wheelbase of design vehicle between consecutive axles and articulation points, m.

As shown in the formula above, the travelled way width needed on a curve (Wc) has

several components related to operation on curves, including: the track width of

each vehicle meeting or passing, U; the lateral clearance for each vehicle, C; width of

front overhang of the vehicle occupying the inner lane or lanes, FA; and a width

allowance for the difficulty of driving on curves, Z.

To determine width Wc, it is necessary to select an appropriate design vehicle. The

design vehicle should usually be a truck because offtracking is much greater for

trucks than for passenger cars. The WB-15 design vehicle is considered

representative for two-lane open-highway conditions.

The track width (U) for a vehicle following a curve or making a turn, also known as

the swept path width, is the sum of the track width on tangent (u) (2.44 or 2.59 m

depending on the design vehicle) and the amount of offtracking.

Lecture 04 43


Dr. Firas Asad

FA = + (+ ) - R A = front overhang of inner lane vehicle, m; L = wheelbase of single unit or tractor, m.

Z = 0.1 ( V/ )

V = design speed of the highway, km/h.

The offtracking depends on the radius of the curve or turn, the number and location

of articulation points, and the lengths of the wheelbases between axles. The track

width on a curve (U) is calculated using the equation:

The lateral clearance allowance, C, provides for the clearance between the edge of

the travelled way and nearest wheel path and for the body clearance between

vehicles passing or meeting. Lateral clearance per vehicle is assumed to be 0.6, 0.75,

and 0.9 m for tangent lane widths Wn, equal to 6.0, 6.6, and 7.2 m respectively.

The width of the front overhang (FA) is the radial distance between the outer edge of

the tire path of the outer front wheel and the path of the outer front edge of the

vehicle body. For curves and turning roadways, FA depends on the radius of the

curve R, the extent of the front overhang of the design vehicle A, and the wheelbase

of the unit itself L. In the case of tractor-trailer combinations, only the wheelbase of

the tractor unit is used.

The extra width allowance (Z) is an additional radial width of pavement to allow for

the difficulty of manoeuvring on a curve and the variation in driver operation. This

additional width is an empirical value that varies with the speed of traffic and the

radius of the curve. The additional width allowance is expressed as:

Lecture 04 44


Dr. Firas Asad

Lecture 04 45


Dr. Firas Asad

C- Application of Widening.

1- On simple (unspiraled) curves, widening should be applied on the inside edge of

the travelled way only. On curves designed with spirals

2- Curve widening should transition gradually over a length sufficient to make the

whole of the travelled way fully usable. Preferably, widening should transition over

the superelevation runoff length, but shorter lengths are sometimes used. Changes

in width normally should be effected over a distance of 30 to 60 m.

, widening may be applied on

the inside edge or divided equally on either side of the centreline. In either case, the

final marked centreline and any central longitudinal joint should be placed midway

between the edges of the widened travelled way.

WB-15 design Vehicle

==============================================

It is suggested that a minimum

widening of 0.6 m be used

Documents

Lec 04 Highway Engineering - Curve Widening