CE 322: TRANSPORTATION ENGINEERING II
Traffic rotaries
Nipjyoti Bharadwaj
Assistant professor, Department of Civil Engineering
Indian Institute of Technology-Guwahati
Advantages
• Traffic flow is regulated to only one direction of movement, thus eliminating severe conflicts
between crossing movements.
• All the vehicles entering the rotary are gently forced to reduce the speed and continue to move at
slower speed. Thus, none of the vehicles need to be stopped, unlike in a signalized intersection.
• Because of the lower speed of negotiation and elimination of severe conflicts, accidents and their
severity are much less in rotaries.
• Rotaries are self governing and do not need practically any control by police or traffic signals.
• They are ideally suited for moderate traffic, especially with irregular geometry, or intersections with
more than three or four approaches.
Disadvantages
• All the vehicles are forced to slow down and negotiate the intersection. Therefore, the
cumulative delay will be much higher than channelized intersection.
• Even when there is relatively low traffic, the vehicles are forced to reduce their speed.
• Rotaries require large area of relatively at land making them costly at urban areas.
• They are not suitable when there is high pedestrian movements.
Guidelines for the selection of rotaries
• Rotaries are suitable when the traffic entering from all the four approaches are relatively equal.
• A total volume of about 3000 vehicles per hour can be considered as the upper limiting case,
and a volume of 500 vehicles per hour is the lower limit.
• A rotary is very beneficial when the proportion of the right-turn traffic is very high; typically if it is
more than 30 percent.
• Rotaries are suitable when there are more than four approaches or if there is no separate lanes
available for right-turn traffic.
• Rotaries are ideally suited if the intersection geometry is complex.
Traffic operations in a rotary
• 1. Diverging: It is a traffic operation when the vehicles moving in one direction is separated into
different streams according to their destinations.
• 2. Merging: Merging is the opposite of diverging. Merging is referred to as the process of
joining the traffic coming from different approaches and going to a common destination into a
single stream.
• 3. Weaving: Weaving is the combined movement of both merging and diverging movements in
the same direction.
Design elements
• The design elements include
• design speed,
• radius at entry, exit and the central island,
• weaving length and width,
• entry and exit widths.
• In addition the capacity of the rotary can also be determined by using some
empirical formula.
Design of a rotary
Design speed
• All the vehicles are required to reduce their speed at a rotary. Therefore, the design speed
of a rotary will be much lower than the roads leading to it.
• Although it is possible to design a roundabout without much speed reduction, the
geometry may lead to very large size incurring huge cost of construction.
• The normal practice is to keep the design speed as 30 and 40 kmph for urban and rural
areas respectively.
Entry, exit and island radius
• The radius at the entry depends on various factors like design speed, super-elevation,
and coefficient of friction.
• The entry to the rotary is not straight, but a small curvature is introduced. This will force
the driver to reduce the speed.
• The entry radius of about 20 and 25 metres is ideal for an urban and rural design
respectively.
Entry, exit and island radius
• The exit radius should be higher than the entry radius and the radius of the rotary island
so that the vehicles will discharge from the rotary at a higher rate.
• A general practice is to keep the exit radius as 1.5 to 2 times the entry radius.
• However, if pedestrian movement is higher at the exit approach, then the exit radius could
be set as same as that of the entry radius.
Entry, exit and island radius
• The radius of the central island is governed by the design speed, and the radius of the
entry curve.
• The radius of the central island, in practice, is given a slightly higher radius so that the
movement of the traffic already in the rotary will have priority.
• The radius of the central island which is about 1.3 times that of the entry curve is
adequate for all practical purposes.
Width of the rotary
• The width of the carriageway at entry and exit will be lower than the width of the
carriageway at the approaches to enable reduction of speed.
• IRC suggests that a two lane road of 7 m width should be kept as 7 m for urban roads and
6.5 m for rural roads.
• Further, a three lane road of 10.5 m is to be reduced to 7 m and 7.5 m respectively for urban
and rural roads.
Width of the rotary
• The width of the weaving section should be higher than the width at entry and exit. Normally
this will be one lane more than the average entry and exit width.
𝑒1+𝑒2
• Thus weaving width is given as 𝑊𝑤𝑒𝑎𝑣𝑖𝑛𝑔 = 2
+ 3.5
• Weaving length determines how smoothly the traffic can merge and diverge.
• This can be best achieved by making the ratio of weaving length to the weaving width very
high. A ratio of 4 is the minimum value suggested by IRC.
• Very large weaving length is also dangerous, as it may encourage over-speeding.
Capacity
• The capacity of rotary is determined by the capacity of each weaving section.
Transportation road research lab (TRL) proposed the following empirical formula to find
the capacity of the weaving section.
𝑒1 +𝑒2
• where e is the average entry and exit width 2
, w is the weaving width, l is the length
of weaving, and p is the proportion of weaving traffic to the non-weaving traffic.
Capacity
• Figure shows four types of movements at a weaving section, a and d are the non-weaving
traffic and b and c are the weaving traffic.
𝑏+𝑐
• P=𝑎+𝑏+𝑐+𝑑
Question
• The width of a carriageway approaching an intersection is given as 15 m. The entry and
exit width at the rotary is 10 m. The traffic approaching the intersection from the four sides
is shown in the figure below. Find the capacity of the rotary using the given data.
Thank You
