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Incorporating Truck Analysis into the Highway Capacity Manual (2014)

Chapter: Section 11 - Conclusions and Recommendations

« Previous: Section 10 - Predicting the Effect of Trucks on Capacity
Page 134
Suggested Citation:"Section 11 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Truck Analysis into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22311.
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Suggested Citation:"Section 11 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Truck Analysis into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22311.
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Suggested Citation:"Section 11 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Truck Analysis into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22311.
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Suggested Citation:"Section 11 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Truck Analysis into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22311.
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Suggested Citation:"Section 11 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Truck Analysis into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22311.
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Page 139
Suggested Citation:"Section 11 - Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Truck Analysis into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22311.
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Page 139

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134 S e c t i o n 1 1 This section presents the conclusions and recommendations from NCFRP Project 41. 11.1 Conclusions This research project reached the following conclusions regarding better incorporating trucks into Highway Capacity Manual (HCM) analysis. • Literature Review: – Most countries use passenger car equivalents (PCEs) like the U.S. HCM to convert trucks in the traffic stream into the equivalent number of passenger cars before computing capacity and speed. China, Indonesia, Singapore, Thailand, and Canada subdivide trucks into three or four subtypes (unlike the United States, which uses only a single truck category). – The literature identified truck weight-to-horsepower ratio as a significant factor for affect- ing freeway performance on extended grades. Length was identified as a possible factor affecting truck PCEs on level terrain. Position in queue was identified as a significant factor affecting saturation flow rates on signalized intersection approaches. – Previous research into truck level of service (LOS) from the point of view of truck drivers identified several physical attributes of the facility that affect their perceived quality of ser- vice. These include the quality of the ride and ease of driving (pavement smoothness, fewer maneuvers required, and ease of maneuvers). This research consequently recommended that speed variance and pavement quality be used as measures of truck LOS on freeways and urban streets. For urban streets, additional LOS measures were also identified: ease of turning maneuvers and traffic density. • Public Agency Perspectives: – The majority of public agencies interviewed use HCM methods to evaluate highway per- formance. The second most commonly used tool is microsimulation, followed by FHWA’s Freight Analysis Framework (which uses the area-wide planning method from the 2000 edition of the HCM). – There is a strong preference among the agencies interviewed for a truck LOS methodology that they can use for ranking goods-movement investments and evaluating general highway capacity investments. – The agencies believe that truck LOS should be sensitive to travel time reliability, traffic congestion, and average speed. • Shipper and Carrier Perspectives: – The interviews and survey of shippers and carriers found that freight decisionmaking is complex and often varies by establishment. In addition, the criticality of travel time and on-time delivery varies by a factor of 10 depending on the cost of the material being hauled and the distance hauled (travel time). Lower-valued goods hauled for longer distances (or times) have the lowest value of time. Conclusions and Recommendations

conclusions and Recommendations 135 – In general, travel time, cost, and reliability (on-time performance) are the key determinants of route selection. Local laws, long-term contracts between shippers and receivers, the type of goods being shipped, transportation costs and travel times, and logistics supply chains all impact the relative importance of these attributes in decisionmaking of shippers, receivers, and carriers. • Vehicle Classification System: – The FHWA axle- and trailer-based vehicle classification system is the appropriate founda- tion for classifying trucks for the purpose of highway capacity analyses. While there are still significant variations in vehicle performance within each of the 13 FHWA vehicle classes, the FHWA system is greatly superior to the current 4-class system in the HCM (i.e., passenger cars, buses, RVs, and trucks). The FHWA system also has the significant advantage of being a nationally established consistent system for which weigh-in-motion data is already readily available. – The 13 FHWA vehicle classes are more than is really needed for HCM analyses, given that many of the classes account for very small percentages of the vehicle fleet. Consequently, it was concluded that the 13 FHWA vehicle classes should be consolidated into the following 5 HCM classes for the purposes of HCM analyses: 1. Passenger Vehicles (FHWA Classes 1, 2, and 3); 2. Buses (FHWA Class 4); 3. Recreational Vehicles (RVs) (a subcategory within FHWA Class 5); 4. Single-Unit Trucks (FHWA Classes 5–7); and 5. Semitrailer Trucks (FHWA Classes 8–13). • Impacts of Trucks on Other Modes: – The current HCM method for basic freeway segments (Chapter 11) of converting trucks into passenger car equivalents is deficient for predicting automobile speeds and truck speeds on extended upgrades under moderate to high flow conditions (3% or greater grades extending over 1 mile, with 5% or more trucks in the traffic stream, under volume/capacity ratios in excess of 0.30). – The current HCM method for estimating speeds for urban street segments (Chapter 17) is insensitive to truck or grade effects. This becomes a significant defect for extended upgrades (3% or more extending over 1 mile). – The current HCM default PCE value of 2.0 for all trucks in roundabouts (Chapter 21) appears to be appropriate. – The HCM’s signalized intersection method (Chapter 18) significantly underestimates the impacts of trucks on saturation flow rates for upgrades in excess of 2%. The relative mix of semitrailer and single-unit trucks had a comparatively minor effect on saturation flow rates (the total percent trucks and grade had significantly greater effects). • Impacts of Other Modes on Trucks (Truck LOS): – Truck LOS should take into account average truck travel times, truck travel time reliability, and cost (where tolls are involved) as well as the truck friendliness of the facility (its ability to safely and legally accommodate all legal vehicles and loads, with as few at-grade railroad crossings as feasible). 11.2 Recommendations This research produced the following recommendations for better incorporating truck analy- sis into the HCM. • Vehicle Classification System: – The 13 FHWA vehicle classes are more than is really needed for HCM analyses, given that many of the classes account for very small percentages of the vehicle fleet. The current

136 incorporating truck Analysis into the Highway capacity Manual single-truck class in the HCM, however, is inadequate to account for the significant per- formance differences between single-unit trucks and semitrailer truck combinations. Con- sequently, it is recommended that trucks in the HCM be split into two vehicle classes, resulting in the following five HCM classes for the purposes of HCM analyses: 1. Passenger Vehicles (FHWA Classes 1, 2, and 3); 2. Buses (FHWA Class 4); 3. RVs (a subcategory within FHWA Class 5); 4. Single-Unit Trucks (FHWA Classes 5–7, excluding RVs); and 5. Semitrailer Trucks (FHWA Classes 8–13). • Impacts of Trucks on Other Modes: – NCFRP Project 41 was able to make significant advances in developing an improved HCM method for estimating automobile and truck speeds on extended upgrades (see Section 8). A preliminary set of equations was developed for predicting speeds. Further research on these speed prediction methods is given in Appendix D. However, as described under “fur- ther research,” further testing and validation is required before these methods can be rec- ommended to replace the existing HCM Chapter 11 methodology. – The project also made significant progress on developing a truck- and grade-sensitive speed estimation method for arterial street segments (HCM Chapter 17) (see Section 9). How- ever, further research will be needed to integrate the new method with the current HCM Chapter 17 method. – For signalized intersections (HCM Chapter 18), is recommended that the current heavy- vehicle and grade adjustment factors in the saturation flow equation be replaced with a single combined factor that better accounts for the synergistic effects of heavy vehicles on signalized intersection approaches with steep upgrades (in excess of 2%). • Impacts of Other Modes on Trucks: – This research developed a recommended truck LOS model based on mean speed, travel time reliability, and added cost associated with tolls (see Section 6). 11.3 Recommended HCM Implementation Plan This section provides a recommended HCM Implementation Plan for moving the results of the research into practice. 11.3.1 Incorporation into NCHRP Project 3-115 (HCM Update) This research developed, described, and demonstrated new methods for evaluating the effects of highway and street facility performance on trucks (truck LOS) and the effects of trucks on other modes (truck PCEs). • Appendix F provides the recommended edits to the 2010 edition of the Highway Capacity Manual. A computational engine was developed for the truck LOS model. • Appendix E provides a User’s Guide for the computational engine. Together, these two products—Appendixes E and F—will facilitate the incorporation of the results of this research into the next update of the HCM, currently being accomplished under NCHRP Project 3-115. Other aspects of the NCFRP Project 41 research (Sections 8, 9, and 10 related to truck speeds and PCEs on extended grades) would involve major changes to current HCM procedures and therefore must wait until further research can be conducted to better define and validate the new methods.

conclusions and Recommendations 137 11.3.2 Expected Audience/Market for Research Product The expected audience for the research product is the transportation engineering and plan- ning professional community involved in the planning and prioritization of all highway and street improvements. This includes private consultants or employees working for state DOTs, MPOs, cities, and counties. FHWA will also be a user of the products in their role as evaluators or advisors to the development of transportation plans and investment programs. 11.3.3 Possible Impediments to Successful Implementation Lack of awareness and knowledge of the new truck analysis methods is the greatest potential impediment to the successful adoption and implementation of the research product. A second- ary potential impediment would be the lack of software to facilitate application of the recom- mended truck analysis procedures. 11.3.4 Likely Institutional Leaders in Application The likely institutional leader for gaining acceptance of the new methodology for evaluat- ing truck LOS in U.S. practice is FHWA. By adopting the approach as a recommended or required analytical approach for analyses conducted in support of federal funding appli- cations for highway improvement projects, FHWA would go a long way toward securing national acceptance of the concept. State DOTs and MPOs can be leaders in adopting the new method. The Institute of Transportation Engineers (ITE) can promote the new method through training classes. 11.3.5 Activities for Successful Implementation Inclusion of the new methodology in future editions of the HCM would go a long way to suc- cessfully implementing the results of this research. However, just being in the HCM does not ensure actual use of the method. NCFRP Project 41 conducted two workshops with public agency personnel to acquaint a core group of professionals with the new truck LOS analysis methods and computational engines implementing the new methods. NCFRP Project 41 also developed a computational engine for the truck LOS model to facilitate the development of commercial software to implement the methods. (The computational engine, with its limited user interface, will not replace the need for some other commercially oriented product to implement the new HCM truck analysis methods.) The computational engine illustrates for software developers how they might program and match the recommended methodology with their programs. Together, the HCM 2010 updates, the computational engine, and the two workshops (already conducted) will greatly facilitate increased awareness of the products of this research. Additional steps that can be taken in the future include the following: • Showcasing the NCFRP Project 41 products at one of the regular FHWA “Talking Freight” webinars (currently included in Task 11 of NCFRP Project 41) will greatly increase awareness of the new truck analysis methodology and new HCM chapter. • Additional workshops on truck LOS after the publication of the HCM 2010 updates (not included within the current scope for this research project) would greatly increase awareness of the truck LOS model. • A series of papers and presentations prepared by key research team leaders for presentation at ITE and TRB annual meetings will help generate interest by agencies in the method.

138 incorporating truck Analysis into the Highway capacity Manual 11.3.6 Criteria for Determining Progress and Success The key criteria of success will be the adoption of the analysis methods developed in this proj- ect by public agencies, inclusion of the methodologies in FHWA guidance documents and the Highway Capacity Manual, adoption of truck LOS standards by state DOTs and other transpor- tation agencies, and inclusion of the new methods in commercially available highway capacity analysis software. 11.4 Applicability of Results to Practice This research resulted in methodologies and a computational engine for predicting the impacts of highway and street investments on truck LOS, taking into account the relative economic importance to the community. The methodologies will enable agencies to take into consideration truck freight movement effects in their prioritization of transportation improvements. 11.5 Recommendations for Further Research While both the arterial segment and freeway basic segment speed models will require further research before they can be implemented in the HCM, the freeway basic segment model appears to be the most promising topic to follow up on. The underestimation of the deleterious effects on average speeds of extended freeway upgrades is a significant problem when performing eco- nomic analyses of the need for freeway truck climbing lanes. Further research to better define and validate the NCFRP Project 41 freeway speed model for extended grades will significantly improve the investment decisionmaking of state DOTs and other public agencies involved in planning and programming freeway improvements. 11.5.1 Need for Further Research The PCE research portion of this project revealed a serious flaw in the current HCM approach for evaluating long, steep grades (upgrades of 4% as short as 1 mile in length). The current HCM approach converts trucks to PCEs and then uses the passenger car speed flow curve to estimate average speed and density of vehicles, which is used for LOS computations. The research found that there are actually two speed-flow curves on long, steep grades, one for passenger cars and one for trucks. At light flows passenger cars can pass the slower trucks and the facility actually has two speeds, one for trucks and one for passenger cars. As flows or the percent of trucks increase, the two-vehicle class phenomenon breaks down and all vehicles travel at the speed of the trucks (this breakdown happens at moderate truck percentages and volumes, long before the HCM estimated capacity is reached). The effect of this flaw in the current HCM approach is that the need for and the benefits of truck climbing lanes on long, steep grades are significantly underestimated by any analysis employing the 2010 HCM and its earlier editions. The effect becomes evident, especially at-and-near capacity, whenever the grade is steep enough (even for short distances) to force a drop in the truck speeds. It has a major implica- tion for computing the benefit of separating the trucks from the rest of the traffic stream. The effect is significant enough that the current density-based automobile LOS methodology in the HCM for freeway grades may need to be replaced by one that is based on automobile and truck delay.

conclusions and Recommendations 139 11.5.2 Objectives of Further Research The objectives of the further research would be the development and calibration of a new two-vehicle class (passenger cars and heavy vehicles) HCM method evaluating the capacity and performance (speed and density) of freeway segments on long, steep grades. This method will require • Creation of heavy-vehicle speed flow curves (truck, RVs, and bus, as appropriate) for use on long, steep freeway upgrades; • Creation of a methodology for quantifying the impact on passenger car speeds of different mixes and volumes of heavy vehicles on long, steep upgrades that takes into account possible countermeasures such as truck lane restrictions and truck climbing lanes; and • Creation of a procedure combining the separate passenger car and truck speeds into a measure of overall facility performance (such as mean speed and density). 11.5.3 Approach for Further Research The freeway simulation test bed already developed and calibrated under the current research would be employed to develop the new HCM method. In addition, field testing would be con- ducted using data sets already collected for this and other projects. The field data set testing will verify the method’s ability to match actual facility performance. The final report and draft chap- ter produced by this project would then be expanded to incorporate the results of the additional research to develop a two-vehicle class procedure for evaluating the performance of freeway facilities with long steep grades. The objectives of the additional research would be accomplished with the following tasks: • Simulation Experiments: Focus on a richer set of conditions so that HCM models can be developed that predict automobile and heavy-vehicle speeds and densities for a variety of upgrades and grade lengths, taking into account interactions between the vehicle types for a comprehensive set of potential field conditions. This task includes preparing the simulation models for the broader set of tests, conducting the simulations, and analyzing the results. • Model Development: Refine and extend the limited set of HCM predictive models for pas- senger car and truck speed and density that came out of the research to date. Determine the interaction and interference effects between vehicle types. Develop a method to predict com- bined average speed and density. Compare the new HCM predictive models with the original simulation results and assess their performance. • Case Studies: Examine a larger set of field settings using data sets already available to the research team where there are significant grades, truck percentages, and so forth so that the predictions of the models can be checked, validated, and further refined. This task includes assembling the data, preparing the models, and conducting the analyses. • Final Report: Expand the final report to describe and present the findings from the experi- ments and case studies. This task includes preparing the report. • Draft HCM Materials: Expand the draft truck analysis chapter for the HCM to include the new methodology for evaluating truck and passenger car speeds and densities on long steep upgrades and predicting the effects of truck lane restrictions and truck climbing lanes on freeway segment performance. Advise the HCM 2010 Update contractor on inserting the new material into the body of the HCM.

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TRB’s National Cooperative Freight Research Program (NCFRP) Report 31: Incorporating Truck Analysis into the Highway Capacity Manual presents capacity and level-of-service techniques to improve transportation agencies’ abilities to plan, design, manage, and operate streets and highways to serve trucks. The techniques also assist agencies’ ability to evaluate the effects of trucks on other modes of transportation.

These techniques are being incorporated into the Highway Capacity Manual, but will be useful to planners and designers working on projects with significant truck traffic.

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