Turning movements
In situations where the turning traffic is unopposed, saturation flows will
decrease as the proportion of turning traffic increases. Where turning traffic
is opposed, the saturation flow will depend on the number of gaps in the
opposing traffic flow together with the amount of storage space available to
those vehicles making this traffic movement.
The equations derived by Kimber et al. (1986) for predicting saturation flow are:
Unopposed traffic streams
The saturation flow is given by:
S 1 =(S 0 - 140 dn)/(1 +1.5f/r) pcu/hr (5.24)where
S 0 = 2080 - 42 dg ¥G+100(w -3.25) (5.25)and
dn=1 (nearside lane) or 0 (non-nearside lane)
f =proportion of turning vehicles in the lane under scrutiny
r =radius of curvature of vehicle path, metres
dg=1 (uphill entry roads) or 0 (downhill entry roads)
G=percentage gradient of entry road
w=entry road lane width, metres.
Opposed traffic streams
In this instance, the saturation flow in a given lane for right-turning opposed
streams is given by:
S 2 =Sg+Sc(pcu/hr) (5.26)The two components of this equation are computed as follows.
(1) Sg
Sgis the saturation flow occurring during the ‘effective green’ time within the
lane of opposed mixed turning traffic.
Sg=(S 0 - 230)/(1 +(T -1)f) (5.27)
T = 1 +1.5/r +t 1 /t 2 (5.28)t 1 =12X 02 /(1 +0.6(1 -f)Ns) (5.29)
X 0 =q 0 /ln 1 s 0 (5.30)t 2 = 1 - (fX 0 )^2 (5.31)The Design of Highway Intersections 135