Vrbc=Visibility to the right from a point 10 m back from the give-way line for
vehicles making the b-cmanoeuvre, metres
Vrcb=Visibility to the right, along the major road for traffic crossing traffic per-
forming thec-bmanoeuvre, metres
Wcr=Width of the central reserve (only for dual carriageways), metres
Y =(1 -0.0345W)
W =Total major road carriageway width, metres.Similar equations exist for estimating capacities at staggered junctions and cross-
roads (see Semmens, 1985).
The determination of queue lengths and delays is of central importance to
assessing the adequacy of a junction. When actual entry flows are less than
capacity, delays and queue sizes can be forecast using the steady state approach.
With this method, as demand reaches capacity, delays and therefore queue
lengths tend towards infinity.
The steady state result for the average queue length L is:L =r+Cr^2 /(1 -r) (5.4)where
C is a constant depending on the arrival and service patterns; for regular vehicle
arrivals C =0, for random arrivals C =1. In the interests of simplicity, the latter
case is assumed.r =flow (l) ∏capacity (m)Therefore, Equation 5.4 can be simplified as:L =r/(1 -r) (5.5)Thus, as rÆ1, L Æ•In reality, this is not the case with queue lengths where the ratio of flow to capac-
ity reaches unity. Thus, at or near capacity, steady state theory overestimates
delays/queues.
On the other hand, within deterministic theory, the number of vehicles
delayed depends on the difference between capacity and demand. It does not
take into account the statistical nature of vehicle arrivals and departures and
seriously underestimates delay, setting it at zero when demand is less than or
equal to capacity.
The deterministic result for the queue length L after a time t, assuming no
waiting vehicles, is:L =(r-1)mt (5.6)Thus, as rÆ1, L Æ 0At a busy junction, demand may frequently approach capacity and even
exceed it for short periods. Kimber and Hollis (1979) proposed a combination112 Highway Engineering