Highway Engineering

The maximum ranges together with those recommended for design are as shown
in Table 5.3.

The Design of Highway Intersections 127

Symbol Description Allowable range Recommended range

e Entry width 3.6–16.5 m 4.0–15.0 m

v Approach half-width 1.9–12.5 m 2.0–7.3 m

l¢ Average flare length 1 m to infinity 1–100 m

S Flare sharpness Zero–2.9 m —

D Inscribed circle diameter 13.5–171.6 m 15–100 m

f Entry angle 0–77° 10–60°

r Entry radius 3.4 m to infinity 6–100 m

Table 5.3Geometric parameters for roundabouts

Example 5.4 – Assessing the ratio of flow to capacity for each of the

entry points to a 3-arm roundabout

Figure 5.25 indicates a set of design reference flows for the evening peak hour

at a proposed roundabout intersection. All flows are in vehicles per hour.

250

650 350

150

200 200

C A N

B

Design reference flow

When analysing the capacity of a roundabout intersection, the capacity of each
of the entry arms is assessed and compared with the traffic flow expected at peak
hours within the design year. This ratio of flow to capacity (RFC) for each traffic
movement, in the same manner as for priority junctions, directly indicates
whether the roundabout will operate efficiently in the chosen design year.
TA 23/81 states that if an entry RFC of 70% occurs, queuing will be avoided
in 39 out of 40 peak hours. A maximum RFC of 0.85 is recommended as this
will result in an intersection whose provision is economically justified yet will
not cause excessive delay and disruption.
Again, as stated earlier, use of the manual procedure will require that all
design reference flows (DRF) are multiplied by 1.125 in order to allow for
short-term variations in traffic flow at the roundabout within the peak hour.

Figure 5.25Design reference flows.

Contd