Click for next page ( 24

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 23
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): Page 24

OCR for page 23
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 Page 25

OCR for page 23
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 Page 26