There are many warrants for traffic signals, but no definitive warrant for roundabouts. Some
references (e.g. TRB 2000) give guidelines, and refer to the capacities of roundabouts and TWSC
(two-way stop control), but capacities address the upper limit of the control device, not the
lower values which would justify, or warrant, a change in control. Other references refer to the
need for a roundabout warrant but do not develop one. One paper by Sampson and Meijer
(1999) discussed potential warrants, but no recommendation was made.
In this paper the role of roundabouts and mini-circles in filling the large gap between where
stop control is no longer ideal, and where traffic signals are required, is examined.
It is found that there are many considerations in addition to volume or delay that need to be
taken into account when deciding on whether to implement roundabout type control, but as
a rule of thumb, if the average queue at a stop control exceeds two (2Q) during the peak hour,
roundabouts will improve the traffic flow, and if the queue totals around ten or more (10Q),
then the roundabout is likely to have reached its capacity.