1 - HKU Libraries

T.P.D.M.V.2.4
4.5.16 Segregated Left Turning Lanes
4.5*16*1 Segregated left turning lanes should be considered when more
than 50% of the entry flow, or more than 300 vehicles per hour
in the peak hours, turn left at the first exit. For design
purposes a maximum capacity of 1800 pcu/hr may be assumed.
Segregated left turning lanes should then be omitted from the
calculation of entry capacity.
4.5.16.2 In segregated left turning lanes vehicles are channelled into
the left hand lane by lane arrows and markings, supplemented by
advance direction signs, and vehicles proceed to the first exit
without having to give way to others using the roundabout.
Segregation by road markings, as shown in Diagram No. 4.5*16.1
is more common than physical segregation as shown in Diagram No.
4.5*16.2. The former method is less effective however because
it is subject to abuse.
4.5.16.3 Segregated left turning lanes should be about 3.5m wide and not
less than 3m. On smaller radius curves widths should comply
with the requirements set out in Table 4.3.15.1, but should not
be so wide as to induce high speeds,
4.5.16*4 Where road markings are used to create the lane segregation
double white lines may be appropriate for the purpose.
4.5.16.5 The merging between vehicles from a segregated left turning lane
and other vehicles exiting from the roundabout should take place
within 50m of the roundabout, where speeds are still
comparatively low. Ideally there should not be a forced merge*
However, running the two streams alongside each other is only
possible where the exit link can provide two lanes in the same
direction. In other cases the segregated left turning traffic
must merge with the other stream, giving way where necessary.
This merging length should be at least 10m Examples of typical
urban situations are shown in Diagram Nos. 4.5.16.2 and
4.5.16.3.
4.5.16*6 Segregated left turning lanes should not be used when vehicular
accesses to adjacent properties exist along the length of the
lane.