1.1           Roundabouts


O’Flaherty 97 page 369, O’Flaherty 86  page 350


Old type was called conventional roundabout


There are 3 types of roundabout:


1.     Normal roundabout

It has a one – way circulating carriageway around a kerbed central island. The entries may or may not have flared approaches. (Preferred to be flared)

In Britain the central island is normally 4m or more in diameter and the entries are flared.


2.     Mini roundabout

It has a one – way circulating carriageway around a flush or slightly raised circular marking less than 4m in diameter. The entries may or may not have flared approaches.


3.     Double roundabout

It has two normal or mini roundabouts either contiguous (having a common border) or connected by a central link road or kerbed island.

Other types of roundabout:

-         Signalised roundabout that has traffic signals installed on one or more of the approach arms.

-         Grade separated roundabouts which has at least one entry road via an interconnecting slip road from a road at a different level, e.g. underpasses, flyovers.


Mini roundabouts can be particularly effective in :

-         Improving existing urban junctions that experience capacity and safety problems.

-         It is only used when speed limit is < or = 48 km/h.


Double roundabouts have a number of special applications:

1.     at an awkward site such as a scissors junction

2.     at an existing staggered junction

3.     at overloaded single roundabouts.

4.     at junctions with more than four entries.


Capacity Determination:


a)     Conventional weaving roundabouts:

No longer used.

b)    Roundabouts under offside-priority control:

In 1963 the offside-priority rule was introduced in the UK. According to this rule vehicles entering the roundabout have to give way to the left “international system”.

-         Therefore, each entry functions as a priority junction.

-         Circulating vehicles take precedence.

Recent research studies:

Qe (entry flow) is linearly dependent upon circulating flow crossing the entry (Qc).


Factors affecting the capacity of a roundabout:

  1. Inscribed circle diameter D (m)

  2. width of flare of each entry. e (m),
  3. Approach width v (m)
  4. effective length over which flare is developed l (m)
  5. entry angle (f)
  6. entry radius (r) (m).


See Table 20.2, O’Flaherty  p 374

Qe      =       entry flow in pcu/h

Qc      =       circulating flow across the entry in pcu/h










Note:  take 1HGV = 2 pcu.

K       =       constant

F       =       the intercept

fc       =       the slope



K, F and fc are factors that depend on the geometry of the roundabout

See O’Flaherty 97 page 374, Table 20.2 and Figure 20.8

and  O’Flaherty86  p 354, Table 6.17 and Fig 6.6

for definition of e, v, l’, S, D, r and f.

K = 1-0.00347(f -30)- 0.978[(1/r)-0.05]

F = 303 X2,

fc = 0.210 tD (1+0.2 X2) 



X2= v + (e-v) / (1+2S)

S=1.60 (e-v)/l

tD = 1 + 0.5/(1+M)

M= exp[(D-60)/10]




The Ration of Flow to Capacity

The (RFC) is an indicator of the likely performance of a junction under a future year traffic loading.

Due to site-to-site variations, there is a standard error of prediction of the entry capacity by the formulae of ±15% for any site.

Thus RFC = Flow / Capacity should be < 85%

For grade – separated roundabouts


Example 1:

Find the capacity of  arm A if :

     Flow ( Qc ) =230 veh/h
     Slope( fc )  = 0.8

     Intercept (F) = 2371


                                                                      (Answer:  Qe=2165pcu/h)


If the demand flow is 1900 pcu/h. Find the RFC and comment on the results.

                                        (Answer:  RFC=0.88 _ 88% > 85 , Not good)


Example 2:

Calculate the reserve capacity for the east approach of a roundaboutand check the capacity

 given the following geometric and flow data:

D = 50m

e = 8.2m




v = 7.5m

l’ = 22m




r = 23m




f = 30°

Demand flow










-         Given that the design year flow is expected to increase 30%.

-         Examine the capcity in the design year

-         Suggest any improvements to the design of the roundabout if necessary

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