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· Right-turn flow rate on the approach (only for proposed shared
through/right ATLs), in vehicles per hour; also right-turn flow rates into
downstream driveways if those are available and deemed to be significant
· Effective green time for the approach, in seconds during the peak 15-
minute period
· Intersection cycle length, in seconds during the peak 15-minute period
· Adjusted saturation flow rate for through and right-turn movements on
the approach (using HCM 2010 methods), in vehicles per hour.
ATL VOLUME ESTIMATION
This section describes a step-by-step analytical method to predict the
through-movement volume that will use an ATL. The method is based
principally on the demand-to-capacity relationship of an intersection approach
without an ATL, and estimates the expected volume in the ATL based on various
parameters. The base estimation method is founded on models built from field
data collected on ATL use. Separate prediction models were developed for one-
CTL and two-CTL approaches.
The field model estimates are constrained by upper bound estimates from
the HCM 2010 model, which specify the maximum through volume to be
expected in any exclusive or shared lane, where through traffic has a choice of
lanes. The upper bound estimate also guarantees consideration of right-turn
traffic effects, even for those cases where field observations did not show an
impact due to low right-turning movements. While the method predicts ATL
volume, the utilization percentage can be calculated from the results if desired.
Approaches with One CTL
A key parameter for the analysis of an ATL facility with one existing CTL is
XT, the ratio of through-movement demand to capacity, also listed earlier in
Equation 3-1:
Equation 3-2
where:
VT = 15-minute through-movement demand flow rate on the
approach, expressed in vehicles per hour;
ST = Adjusted through saturation flow rate per lane on the
approach, in vehicles per hour;
g = Effective green time for the approach, in seconds; and
C = Intersection cycle length, in seconds.
In Equation 3-2, VT is the total through demand flow rate, whereas ST is the
per-lane saturation flow rate of the CTL. Once XT is computed, the through-
movement flow rate in the ATL can be predicted using Equation 3-3 (R2 = 0.781):
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Equation 3-3
where:
VATL = The predicted through-movement flow rate in the ATL (in
vehicles per hour), and all other variables are as
previously defined.
The remaining flow rate in the continuous lane, VCTL , is obtained by
subtracting VATL from VT. This method can be used to estimate ATL use on
approaches with one CTL in situations when the ATL will be an exclusive lane
and when it will be a shared lane with right turns. Equation 3-3 does not contain
a right-turn volume variable, because the measured right-turn volumes in the
field were not high enough to impact the ATL through volume for observed
shared ATL sites. This does not mean that right-turn effects will be ignored. In
fact, those will be accounted for in the estimation of an upper-bound flow rate
using HCM 2010 methods.
ATL Approaches with Two CTLs
For an approach with two CTLs and a proposed shared ATL, an additional
parameter XR (the right-turn volume-to-capacity ratio) must be estimated as
given in Equation 3-4 (R2 = 0.768):
Equation 3-4
where:
VR = Right-turn flow rate for the proposed shared ATL (this
term could include right-turn traffic entering the
downstream driveways if that flow rate is available and
deemed to be significant); and
SR = Adjusted right-turn saturation flow rate in the proposed
shared ATL (usually defaults to 0.85 ST).
In the case of an exclusive ATL, XR is set to zero in Equation 3-5. The
through-movement flow rate (in vehicles per hour) in the ATL can then be
predicted using Equation 3-5:
Equation 3-5
Similar to the one-CTL case, VT represents the total approach through
volume. Upon computing VATL, the remaining volume in both continuous lanes
(VCTL) is again obtained by subtracting VATL from VT. This research did not show
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evidence of uneven lane utilization across the two continuous lanes, and VCTL is
therefore assumed to be divided equally across the two CTLs.
Upper-Bound Values for ATL Use
Regardless of the predicted ATL flow rate derived from Equations 3-3 or 3-5,
when given a choice, drivers will generally seek the lane that will minimize their
own queue position service time. This upper bound on the typical through flow
rate for an ATL is best represented by the equal v/s (volume -to-adjusted
saturation flow rate) approach adopted in the HCM 2010 (2). It essentially states
that through traffic on an approach will divide itself across several eligible lanes
in a manner that equalizes all lane v/s ratios serving the through traffic.
Therefore, if an exclusive ATL is contemplated, the upper bound for the ATL
through flow rate for the single CTL case can be estimated using Equation 3-6:
Equation 3-6
VATL,MAX
In the case of two CTLs, the upper bound is computed using Equation 3-7:
Equation 3-7
VATL,MAX =
where:
VATL,MAX = Upper bound for ATL through flow rate, in vehicles per
hour; and
fLU = HCM 2010 lane utilization factor (see HCM 2010; Exhibit
18-30 for default values).
In the case of a shared through-right ATL, the lane utilization factor is not
applicable because lane choice is governed by the possible impedance caused by
right turns in the shared lane. Instead, the upper bound for ATL flow rate is
estimated on the basis of the equal v/s concept, using Equation 3 -8:
Equation 3-8
where:
N = Number of CTLs and shared ATLs on the proposed
approach,
VR = The right-turn flow rate from the shared ATL (in vehicles
per hour including possibly right turns onto downstream
driveways), and
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