1 - HKU Libraries

T.P.D.M.V.2.4
4.2.5 Delay
4.2.5.1 Obviously delay is closely related to capacity and design flow as described in the previous
two sections. As the design flow approaches the capacity of the junction delays will
increase. In this introductory section it is considered worthwhile to briefly examine aspects
of delay at the different junction types.
4.2.5.2 At priority junctions delay will be experienced only by the minor road traffic. Major road
traffic will flow virtually unimpeded almost to the point where the major road flow equals
the major road capacity. Before this point is reached however the delays to vehicles on the
minor road will have reached intolerable proportions. Long delays to minor road traffic also
encourage smaller gap acceptance and a consequent increase in accidents.
4.2.5.3 Unlike priority junctions traffic signals can distribute delay fairly so that no particular
movement suffers disproportionate delays. At low levels of flow however, total delay at a
junction will increase when compared with a priority layout. Minimum cycle times are
required by safety considerations and where this cycle time is greater than that warranted on
capacity grounds unnecessary delay results. This problem is particularly prevalent at very
off peak times, such as the middle of the night, despite the facility with all modern
controllers to vary phase and cycle times throughout the 24 hours. Delays can of course be
minimised by linking adjacent traffic signal controlled junctions.
4.2.5.4 With well balanced flows delays at roundabouts will be minimised over a full range of flow
up to levels which are close to capacity. Problems can arise with single predominant flows
causing long delays to those traffic streams across whose entry they pass. Also roundabouts
should be avoided in locations covered by Area Traffic Control.
4.2.5.5 Delays to major through movements are abolished completely at grade separated
interchanges while delays to minor turning movements can be minimized by use of
appropriate junction control where the ramps and minor roads intersect.