Horizontal Alignment

The principles in road alignment selection and factors affecting the selection are discussed in Section 8.4, while the horizontal alignment standard to be used is described in this section.

The horizontal alignment should always be designed to the highest standard consistent with the topography and be chosen carefully to provide good drainage and to minimize earthworks. The alignment design should also be aimed at achieving a uniform operating speed.

Therefore the standard of alignment selected for a particular section of road should extend throughout the section with no sudden changes from easy to sharp curvature. Where sharper curvature is unavoidable, a sequence of curves of decreasing radius is recommended.

Near-minimum curves should in particular not be used at the following locations:

• on high fills or elevated structures, as the lack of surrounding objects, reduces the drivers’ perception of the road alignment;

• at or near a vertical curve, especially crest curves, as it would be extremely dangerous, in particular at night time;

• at the end of long tangents or a series of gentle curves; also compound curves, where a sharp curve follows a long flat curve, should be avoided in order not to mislead the driver;

• at or near intersections and approaches to bridges, in particular approaches to single-lane bridges.

The horizontal alignment consists of a series of intersecting tangents and circular curves, with or without transition curves.

Straights

Long straights should be avoided as they are monotonous for drivers and cause headlight dazzle on straight grades. A more pleasing appearance and higher road safety can be obtained by a winding alignment with tangents deflecting some 5–10 degrees alternately
to the left and right.

Short straights between curves in the same direction should not be used because of the broken back effect. In such cases where a reasonable tangent length is not attainable, the use of long, transitions or compound curvature should be considered.

The unfavourable broken back effect may also be improved by the introduction of a sag curve.

The following guidelines may be applied concerning the length of straights:

1. straights should not have lengths greater than (20×V) metres, where V is the design speed in kmh−1;

2. straights between circular curves turning in the same direction should have lengths greater than (6×V) metres, where V is the design speed in kmh−1;

3. straights between the end and the beginning of untransitioned reverse circular curves should have lengths greater than two-thirds of the minimum of the total superelevation run-off (see Chapter 9).

Circular curves:

As a vehicle traverses a circular curve it is subject to inertial forces which must be balanced by centripetal forces associated with the circular path. For a given radius and speed a set of forces is required to keep the vehicle in its path. The radius can be expressed by the formula:

Where:
R=radius of the curve (metres);
V=speed of vehicle (kmh−1);
e=crossfall of road (%) (e is negative for adverse crossfall);
f s =coefficient of side (radial) friction force developed between the tyres and road pavement.

By braking in curves both side and tangential frictional forces are active. The portion of the side friction that can be used with comfort and safety is normally determined as not more than half of the tangential coefficient of friction.

Superelevation

For small radius curves and at higher speeds, the removal of adverse cross fall alone will be insufficient to reduce frictional needs to an acceptable level and cross fall should be increased by the application of superelevation. The maximum value of superelevation is normally set at 7% to take account of the stability of slow, high laden commercial vehicles and the appearance of the road.

However, Overseas Road Note 6 indicates that a superelevation rate of 8% or even 10% may be applied for paved roads under special conditions.

Minimum radius Table 8.1 shows the minimum horizontal curve radii together with the assumed side

friction factors recommended by Overseas Road Note 6 and the SATCC Recommendations for maximum superelevation of 7 and 10%.
The recommended minimum radii of curves below which adverse cross fall should be removed are shown in Table 8.2.

 

The minimum radius for the design of horizontal curves used in Bhutan, which is also shown in Table 8.1, is based both on the superelevation and side friction considerations as well as the need to ensure stopping sight distance in the curve as illustrated in Figure
8.1. The maximum design values of the side friction coefficient used in Bhutan vary from 0.19 at 20–40kmh−1 down to 0.12 at 80kmh−1.

Obstructions

Situations frequently exist where an object on the inside of a curve, such as vegetation, building or cut face, obstructs the line of sight. Where it is either not feasible or economically justified to move the object a larger radius curve will be required to ensure that stopping sight distance is available.

required radius of curve is dependent on the distance of the obstruction from the centre-line and the sight distance as shown in Figure 8.1 and can be derived from the relationship:

where:
M=obstruction to centre-line distance (metres);
R=radius of horizontal curve (metres);
S=stopping sight distance (metres);
110 6000
120 2800 7000

N=driver’s eye and road object displacement from centre-line in metres (can be assumed to be 1.8m).

Length of horizontal curve

It is preferable to design the horizontal alignment using a curve radius close to the desired minimum value for the selected design speed which will minimize the length of the horizontal curve. This type of design provides the maximum length of road where sight distances are not reduced and where overtaking can be carried out.

Previous design methods used longer curves to produce ‘flowing alignments’ and more gentle bends. However, with such designs, sight distances will be restricted on the longer curves and a shorter length of alignment will be available where overtaking is safe.

However, for small changes of direction, it is often desirable to use large radius curves. This improves the appearance of the road by removing rapid changes in the edge profile. It also reduces the tendency for drivers to cut the corners of small radius curves.

The use of long curves with a radius near the absolute minimum should be avoided where possible, as drivers at speeds other than the design speed will find it difficult to remain in the lane. Curve widening reduces such problems (Chapter 9).

Transition curve

The provision of transition curves between tangents and circular curves has the following principal advantages:

1. Transition curves provide a natural easy-to-follow path for drivers, such that the centripetal force increases and decreases gradually as a vehicle enters and leaves a circular curve.

2. The transition between the normal cross slope and the fully superelevated section on the curve can be effected along the length of the transition curve in a manner closely fitting the speed-radius relation for the vehicle traversing it.

3. Where the pavement width is to be widened around a sharp circular curve, the widening can conveniently be applied over the transition curve length, in part, on the outside of the pavement without a reverse-edge alignment.

4. The appearance of the highway is enhanced by the application of transition curves. Transition curves should preferably be used on all superelevated curves for arterial (primary) roads with high design speed. The clothoid which is characterized by having a constantly changing radius is normally considered the most convenient type of transition curve.

For the design of transition curves reference is made to the particular literature on this subject.

Also Read: Types of highway