TYPES OF GRADE INTERSECTIONS:

Intersections are classified into different categories based on the geometry of the intersecting roads and based on the type of channelisation provided.

(a) Based on Geometry of Intersecting Roads: Various types of intersection are shown in Fig 6.8.

Fig 6.8. FORMS OF INTERSECTIONS

(b) Unchannelised Intersections: They are the simplest type but most dangerous and inefficient in traffic operation. There is absolutely no restriction to vehicles to use any part of the intersection area ; there is increase in the maximum conflicting area resulting in more number of accidents unless controlled by a police man. These unchannelised intersections may be either plain or flared (Fig 6.9). In the case of plain intersections no additional pavement width is provided for turning movements and therefore they are the most economical form of intersections. On the other hand , in the case of flared intersections the width of intersecting roads is increased at the junction. The extra widths are usually provided on either side of the intersection carriage way area. Since the paths of turning vehicles are not restricted or controlled , there is possibility for reduction in the number of accidents.

Fig 6.9. UNCHANNALISED INTERSECTIONS

(c) Channelised Intersections: These are achieved by introducing islands into the intersectional area , thus reducing the total conflict area available in the unchannelised intersection. These islands are normally triangular in shape and help to channelise the turning traffic , to control their speed and angle of approach and to decrease the conflict area at the intersection.

Channelisation islands should be so located with respect to the approach that their visibility is clear and the intended path is automatically selected. It is better to provide small number of larger sized islands rather than large number of small sized islands. The minimum desirable size of island is 5 square metres in area , and it is better to have an area of 8m² . These islands also serve as refuge islands for pedestrians and location for other traffic control devices. Channalisation may be either partial or complete with divisional and directional islands and medians (Fig 6.10). From traffic operation point of view there is better control on the traffic entering or leaving the intersection and these are considered superior to all paved types. However , one of the crossing vehicles will have to stop while the other proceeds.

Fig 6.10 CHANNELISED INTERSECTION

(d) Rotary Intersections: It is a channelised road intersection , where all the traffic from approach roads is made to move round a large control island in a clockwise direction before weaving out into their desired direction of road radiating from the rotary. Necessity of

stopping at road intersection is eliminated. All the traffic streams merge into a common stream around the centre island and then diverge out to the desired , radiation road. On rotary , crossing conflicts are completely eliminated (See Fig 6.11).

Design Factors of Rotary:

In designing a rotary the following elements are considered.

(i) Design speed: As the vehicles approach the rotary with reduced speed , rotaries on Indian roads are designed for a speed of 40 kmph in rural areas and for a speed of 30 kmph in urban areas.

Fig 6.11. ROTARY INTERSECTION

(ii) Shape of Central Island: The shape of the central island depends on the number and layout of the intersecting roads around it. The central islands should be without any corners and if corners are provided they should be provided with large radius. The various shapes considered to suit different conditions are circular , elliptical , turbine and tangent shape (Fig 6.12). If all the roads are equispaced around the rotary and carry equal importance , circular central island is the most suitable. When the layout of the intersecting roads is four or more , an elliptical shape is preferred. In such a case , the major axis of the ellipse is along the axis

of the import roads. Turbine shape forces reduction in speed of vehicles and enables speeding up of vehicles going out. At night the head light glow is a limitation of the design.

Fig 6.12. SHAPES OF ROTARY ISLANDS

(iii) Radius of Rotary: If the centre island is circular it will have a constant radius throughout its perimeter. But if it is of any other shape , it will have different radii at different locations.

Super elevation is normally not provided at the road around the centre island and it is only the friction that will be counteracting the centrifugal force.

Hence the radius of the curve ® is given by

R (metres) = V²/ 127 f 6.1

where f is the coefficient of friction taken as 0.43 and 0.47 for speeds (V) of 40 and 30 kmph respectively after allowing a factor of safety of 1.5.

(iv) Weaving Angle and Weaving Distance: The angle between the paths of vehicles entering the rotary from a particular radiating road and that of another vehicle leaving the rotary on an adjacent road is known as weaving angle (Fig 6.11). The value of the weaving angle should be kept small , but not less than 15⁰.

Vehicles entering the rotary from a road and leaving towards another radiating road have to first merge into one way traffic flow in the roatary roadway around the central island and than weave out to diverge from the flow to the required road outlet. The weaving operation including merging and diverging can take place between two channelising islands of adjacent intersecting legs ; and the length of the roadway is known as ‘Weaving length ‘

(Fig 6.11). The recommended values of minimum weaving lengths are 45 m and 30 m for speeds of 40 kmph and 30 kmph respectively. The maximum weaving length may be taken as twice the above minimum recommended weaving length.

(v) Width of Rotary Road Way: The minimum width of the roadway between the edge of the central island and adjoining kerb is the effective width of the rotary or of the weaving section and this by and large determines the capacity of the rotary.

The width of non-weaving section e2of the rotary should be equal to the widest single entry to the rotary and should generally be less than the width of weaving section. The width

‘W’ of the weaving section of the rotary should be one traffic lane wider than the mean width of entry (e1) and nonweaving section (e2). That is

W = (e1+ e2) / 2 + 3.5 m 6.2

(vi) Channelising Islands: These should be provided at the entrance and exit of the rotary to prevent undesirable weaving and turning and to reduce the area of conflict. These islands also help in forcing the vehicles to reduce their speed to the design speed of the rotary and to serve as a convenient place foe erecting traffic signs and as a pedestrian refuge. They have kerbs of 15 to 21 cm high.

(viii) Sight Distance and Grade: The sight distance at the rotary should be as large as possible and in no case less than the safe stopping sight distance for the design speed. It is preferable to locate a rotary on level ground. It may also be located on the area which is on a single plane , with the slope not exceeding 1 in 50 with the horizontal.

(viii) Lighting: The minimum lighting required is one each on the edge of the central island facing each radiating road (points A in Fig 6.11). Additional lights ‘B’ may be provided when the central island is larger than 60m dia. Lights ‘C’ may also be provided near the entrance curves if the pedestrians are large in number.

(ix) Traffic signs: The standard traffic (warning) sings indicating the presence of rotary intersection should be installed at all approaching roads. At night a red reflect or red light is placed at about one metre above the road level on the nose of each directional island and on the kerb of the central island and channelising islands are marked by alternating black and white stripes to improve visibility.

(x) Provision for Cyclists and Pedestrians: Pedestrians cause hindrance to the free flow of vehicular traffic. As such they should be isolated from the vehicular traffic using rotary by providing subways or overbridge , though they prove to be costly.

Recommendation of Traffic Rotaries: Traffic rotaries should be recommended when the total volume from all the incoming intersecting roads exceed 500 vehicles / hour , but not more than 5000 vehicles / hour. Under conditions of mixed traffic , the IRC recommends that traffic rotaries should be installed when the vehicular traffic (motor traffic) is about 50% or more of the total traffic , on all the intersecting roads. If fast right turning traffic is more than 30% of the total traffic , rotary is again justified.

Advantages of Traffic Rotaries: Some of the important advantages of traffic rotaries are:

(i) The traffic handling capacity of a rotary is higher than that of any other type of at-grade intersection.

(ii) It makes all the radiating roads to carry traffic almost to their full capacity.

(iii) It is the safest of all types of at-grade intersections.

(iv) Traffic coming from and turning to any direction gets equal preference.

(v) Operational cost of vehicles is smallest because vehicles do not have to stop and re-start.

(vi) Rotary functions by itself and there is no need of traffic pole or signals to control the traffic.

(vii) Number of accidents and severity of accidents are low because of the relative low speed of vehicles.

(viii) When the number of intersecting roads is 4 to 7 , rotaries can be constructed with advantage.

Disadvantages: Limitations of traffic rotaries are as follows:

(i) Area required for the construction of traffic rotaries is very large and hence may not be available in built-up areas.

(ii) If the number of intersecting roads is more than 7 , traffic rotaries are unsuitable.

(iii) Extra length to be traversed by right turning traffic and crossing traffic is quite large and they may try to violate the traffic regulations by adopting to short cuts.