This document discusses performance measures for two-way, two-lane highways and identifies shortcomings in the current Highway Capacity Manual methodology. It summarizes alternative performance measures that have been suggested in research to address the discrepancies, such as Percent Impeded and Average Travel Speed. The document also reviews empirical models that have been developed to predict level of service based on these alternative measures and identifies additional measures used in different studies and countries. It concludes by recommending further research is needed to determine the best headway threshold for differentiating platoons and to identify the most suitable level of service measure for different road environments as the HCM may not always be applicable.

1. Performance Measures on
Two-Way, Two-Lane Highways

2. HCM Performance Measures
Class I Class II Class III
Source: Manual, H. C. (2010). Highway capacity manual. Washington, DC.

3. Shortcomings of HCM methodology
 Surrogate measure of HCM based on 3-sec headway used for measuring PTF in
the field
 Discrepancies between HCM analytical models and the headway-based
method
 To address this issue researchers suggested alternative performance measures
 Any measures contributing to formation of platoons can be potentially used as
a TWTLHW performance measure.

4. Example of an Alternative Performance
Measure
Al-Kaisy, Freedman (2010)
𝑃𝐼 = 𝑃𝑝 𝑃𝑖
 𝑃𝐼 = 𝑝𝑒𝑟𝑐𝑒𝑛𝑡 𝑖𝑚𝑝𝑒𝑑𝑒𝑑
 𝑃𝑝 = 𝑝𝑟𝑜𝑏𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝑜𝑓 𝑎 𝑣𝑒ℎ𝑖𝑐𝑙𝑒 𝑏𝑒𝑖𝑛𝑔 𝑝𝑎𝑟𝑡 𝑜𝑓 𝑎 𝑝𝑙𝑎𝑡𝑜𝑜𝑛 𝑢𝑠𝑖𝑛𝑔 𝑡𝑖𝑚𝑒
ℎ𝑒𝑎𝑑𝑤𝑎𝑦 𝑑𝑒𝑓𝑖𝑛𝑖𝑡𝑖𝑜𝑛 𝑜𝑓 𝑝𝑙𝑎𝑡𝑜𝑜𝑛
 𝑃𝑖 = 𝑡ℎ𝑒 𝑝𝑟𝑜𝑏𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝑜𝑓 𝑎 𝑣𝑒ℎ𝑖𝑐𝑙𝑒 𝑏𝑒𝑖𝑛𝑔 𝑖𝑚𝑝𝑒𝑑𝑒𝑑 𝑎𝑛𝑑 𝑡ℎ𝑢𝑠 𝑡𝑟𝑎𝑣𝑒𝑙𝑖𝑛𝑔
𝑎𝑡 𝑎 𝑠𝑝𝑒𝑒𝑑 𝑙𝑒𝑠𝑠 𝑡ℎ𝑎𝑛 𝑖𝑡𝑠 𝑑𝑒𝑠𝑖𝑟𝑒𝑑 𝑠𝑝𝑒𝑒𝑑

5. Example of an Empirical Model
Predicting LOS measure
Source: Polus, A., & Cohen, M. (2009). Theoretical and empirical relationships for the quality
of flow and for a new level of service on two-lane highways. Journal of Transportation

6. Performance Measures Identified in the
Literature
Performance Measure Study
Percent Following Al-Kaisy and Durbin (2008)
Percent Impeded Al-Kaisy and Freedman (2010)
ATS and PTSF Romana and Perez (2006)
PTSF (based on queuing theory) Cohen and Polus (2011)
Average Travel Speed
Al-Kaisy and Karjala (2008)
Average Travel Speed of passenger
cars
Average travel speed as a
percentage of free-flow speed
Percent follower
Follower density
Polus and Cohen (2009)
Flow
Freedom of flow
Average platoon length
Traffic intensity
PTSF
ATS Brillon and Weiser (2006)

7. Recommendations for Future Research
 No consensus on what threshold should be used for separating platoons. For
Example:
 HCM 1985, Al-Kaisy and Durbin (2008) : 6 sec
 HCM 2000, Cohen and Polus (2011) : 3 sec
 Research topic: what headway threshold can best differentiate the platoons?
 Different countries use different LOS measures:
 Germany: ATS (Brillon and Weiser (2006))
 South Africa: Follower density (Van As (2003))
 HCM may not be applicable to certain road environments, therefore studies on TWTLHW
should be conducted to find the most suitable LOS measure

8. Thanks For Your Attention