A rotary intersection is a specialised form of intersection-at-grade in which the traffic movies in one direction round a central island. For ‘keep-to-the-left’ rule, vehicles approaching the intersection are forced to move around the central island in an orderly manner and weave out of the rotary into their respective desired directions.
Design Elements of a Rotary:
A number of elements of a rotary are considered herein. The guidelines given are in accordance with the “Recommended Practice for Traffic Rotaries, IRC: 65 – 1976, Indian Roads Congress, New-Delhi, 1976”.
(i) Design Speed:
Various elements such as radii and weaving lengths are governed by the design speed. This should be necessarily lower than the design speed of the intersecting highways so as to keep the dimensions within practical limits.
The IRC recommended values are 40 km/h in rural areas and 30 km/h in urban areas.
(ii) Entry Radius:
This is determined by the design speed, super elevation and coefficient of pavement friction. This should be such as to result in the drivers lowering their speed to the design speed of the rotary.
IRC recommends an entry radius of 20 to 35 m for rural areas and 15 to 25 m for urban rotaries.
(iii) Exit Radius:
This should be larger than that of the rotary island for the drivers to leave the rotary rapidly. The recommended practice is to keep the exit radius 1.5 to 2 times more than that of the entry radius.
However, the exit radius may be kept similar to the entry radius in case the pedestrian traffic across the exit road is high, in order to limit the exit speed and allow for pedestrian safety.
(iv) Size of Central Island:
The radius of the central island depends on the design speed of the rotary; theoretically it should be the same as that of the entry radius. However, in practice, the radius of the central island is kept slightly more than the entry radius; this is meant to slow down the approaching traffic and give a little preference to the traffic already on the rotary.
This purpose is adequately served by making the radius of the central island 1 1/3 times that of the entry curve. (IRC: 651976).
(v) Width of Carriageway at Entry and Exit:
This depends on the design traffic entering and leaving the intersection. A minimum of 5 m, with provision for extra width due to curvature, is recommended for the entry and exit. The recommendations of IRC are given in Table 5.4.
(vi) Weaving Length:
The ease with which the traffic can merge and diverge will be dependent on the weaving length. This is decided based on entry width, the width of weaving section, total traffic volume, and the proportion of weaving traffic in it. If the weaving length is made at least four times the weaving width, direct traffic cuts may be prevented.
IRC: 65-1976 recommends a minimum weaving length of 45 m for a design speed of 40 km/h and 30 m for 30 km/h.
A maximum limit to weaving length is also considered desirable to discourage speeding near the rotary. For this, twice the above values can be taken to be the upper limits.
(vii) Entry and Exit Angles:
A desirable value for entry angle is 60°; the exit angles should be small for smooth exit from the rotary – it can be 30° or less.
(viii) Width of Rotary Carriageway:
Width at entry, width of non-weaving section and width of weaving section are the important elements of the rotary carriageway.
The width of the non-weaving section should be at least equal to the widest single entry into the rotary.
The width of the weaving section should be more than this. The general recommendation (IRC: 65-1976) is-
(ix) External Kerb Line:
The external kerb line of a weaving section should be normally re-entrant – either straight or a large radius curve, similar to exit curves. This tends to eliminate wastage of area near the middle of weaving sections, as it is unlikely to be used by traffic.
(x) Sight Distance:
Stopping sight distance for the adopted design speed should be provided. A rotary should be preferably on a flat area, at any rate not steeper than 4 in 50.
Barrier type kerbs are preferred at the outer edges of a rotary to discourage pedestrians from crossing over.
(xi) Super-Elevation and Camber:
The rotary curvature is opposite to that of entry and exit; so, the super-elevations will be opposite to each other. This causes difficulty to high and heavy vehicles. For the least discomfort, the algebraic difference in the cross-slopes should be limited to about 0.07; channelising islands should be located at the neck. Slight adjustments may be made from the point of view of curvature and speed.
Capacity of a Rotary:
This is directly dependent on the capacity of each weaving section, which is governed by the geometry of the intersections including entrances and exit, and the percentage of the weaving traffic in relation to the total traffic volume.
The Transport and Road Research Laboratory, London, recommends the following formula based on extensive research, which is a modification of the War drop formula.
In this formula, the equivalency factors recommended by IRC (IRC: 65-1976, Recommended Practice for Traffic Rotaries) are given in Table 5.5:
Referring to Fig.5.19:
a = Left-turning traffic moving along left extreme lane
b = Crossing/weaving traffic turning towards right while entering the rotary
c = Crossing/weaving traffic turning towards left while leaving the rotary
d = Right-turning traffic moving along right extreme lane
Eq. (5.2) is valid under the following conditions:
(i) No standing vehicles on the approaches to the rotary.
(ii) Approach gradients do not exceed 1 in 25.
(iii) e/ω should be between 0.4 and 1.0
(iv) ω/l should be between 0.12 and 0.40
(v) p should be between 0.4 and 1.0
(vi) l should be between 18 m and 90 m
Certain adjustments are to be made depending on the geometries of the rotary layout:
1. For entry angle between 0° and 15°, the capacity of the weaving section should be reduced by 5%.
2. For entry angle between 15° and 30°, the reduction in capacity should be 2.5%
3. For exit angle is greater than 60°, the reduction in capacity is 5%.
4. For internal angle greater than 95°, the reduction in capacity is 5%.
5. If the pedestrian flow across an exit is more than 300 per hour, the practical capacity is arbitrarily reduced to one-sixth of the preceding weaving section.
Signals on the approaches can increase the capacity of the rotary and improve the operation.
Guidelines for Justification of Traffic Rotary:
1. Rotaries are considered suitable when traffic volumes of intersecting legs are approximately equal.
2. According to IRC, a total traffic volume of 3000 PCU/hour entering from all legs is the practical capacity justifying a rotary.
3. Rotary is a good option when the intersecting legs are more than four but not more than seven.
4. The lowest traffic volume for which a rotary may be considered is given as 500 PCU/hour
5. A rotary is justified if right-turning traffic is more than 30 percent of all approaching traffic.
6. For mixed traffic, IRC recommends a rotary where intersecting traffic is more than 50 percent of the total approaching traffic or where the volume of right-turning traffic is at least 30 percent of it.
The conventional roundabout with a large diameter central island is not truly self-regulating all the time. Locking up of traffic, which can occur at frequent intervals, can cause serious delays through traffic jams.
In view of this, there was a major change in the design approach of rotaries with the introduction of the priority-from the right rule in the U.K. This rule was expected to get over the drawback of locked roundabouts. The absence of ‘weaving’ under this rule made possible a significant reduction in the size of the central island and the consequent increase in available carriageway space. Increased entry width allows the flow of multiple vehicles from the give- way line and the consequent increase in capacity of the mini-round-about has been an attraction.
The entry taper may be around 1 in 6, while the exit taper could be 1 in 12. To maintain the capacity at the junction, a single-lane approach road should be tapered to give at least three lanes and double-lane approach tapered to at least four lanes at the junction. At the exits, the merging should be from four lanes to two lanes and form double-lane to single lane.
1. A general increase in capacity compared to a conventional rotary or even signalised intersection.
2. A trend of decrease in accident rates.
1. Non-observance of “priority from the right” rule can lead to chaos and accidents.
2. Adequate visibility to drivers is great importance.
3. High standards of road signing and pavement markings are necessary for safe operation.
Advantages of Rotary:
(i) One-way movement ensures smooth traffic flow.
(ii) Traffic proceeds continuously at low speed; this reduces or minimizes frequent stopping and starting.
(iii) Direct conflicts are eliminated because of weaving movements, facilitating the stream of traffic to merge or leave at small angles.
(iv) It automatically enforces a check on the vehicle speed.
(v) All turns can be made with ease
(vi) For moderate traffic, no control by police or traffic signals is needed.
(vii) When five or more intersecting legs are involved, and/or when heavy right-turning traffic is present, a rotary offers the best solution.
(viii) A rotary has the maximum possible capacity of all at-grade intersections,
(ix) A rotary, when designed properly, has good safety record.
Disadvantages of Rotary:
The following are the disadvantages of a rotary:
(i) A rotary requires a large area, which is difficult to get in built-up/urban areas.
(ii) In case the pedestrian traffic is large, police control becomes necessary.
(iii) A rotary is unsuitable when the angle of intersection is acute.
(iv) The distance to be covered to reach one leg of the intersection from another becomes more, except in the case of simple left turns.
(v) It is difficult operate when the intersecting legs are more than five or six.
(vi) It is not desirable to provide rotaries at close intervals on important highways.
(vii) Rotaries require large area on high-speed roads; hence, they do not offer the best or economical solution in such cases.
(viii) When the traffic flow reaches capacity, ‘locking up’ of the vehicles may result when smooth weaving is disturbed for any reason. The whole traffic comes to a stand-still requiring the traffic police to restore order,
(ix) The central island, the entrances and exits need good lighting for safe operation during nights; warning and directional signs needed. These requirements tend to make rotaries expensive,
(ix) The rotary cannot be justified for low traffic volumes in rural areas; also, stage development is difficult. (‘Stage development’ means improving the geometric elements of a road such as the width of carriageway in stages as traffic needs grow.)