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Incorporating Truck Analysis into the Highway Capacity Manual (2014)

Chapter: Section 5 - Recommended HCM Truck Classification Scheme

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Suggested Citation:"Section 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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 5 - Recommended HCM Truck Classification Scheme." 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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46 This section reviews the current national and statewide truck classification schemes and recom- mends an appropriate classification scheme for use in the analysis of the effects of trucks on other modes and the effects of other modes on the quality of service experienced by trucks on the highways. Two characteristics of trucks are of primary importance when predicting the effect of trucks on facility performance and vice versa: truck length and the ratio of weight to power.4 There are numer- ous truck classification schemes currently in use in the United States, and each was developed for a different purpose. The intent of this section is to present a method for transforming these different classifications into a set of classifications that can best be used in the Highway Capacity Manual (HCM) to predict truck performance. Exhibit 17 shows the steps that may be required to transform one or more of the existing national classification schemes into a scheme useful for HCM analysis. Another issue relevant to the objectives of NCFRP Project 41 is to provide guidance on analyz- ing highway performance measures that are of direct relevance to shippers. The main thrust of HCM analyses is on average peak period operational measures for a given set of conditions; how- ever, as will be discussed later, reliability is a key concern for many shippers. This is particularly true of those involved with just-in-time inventories or high time value cargos. The rest of Section 5 discusses the following topics: • Overview of existing truck classification schemes. This provides a review of current national classification schemes and some variations on these. • Determinants of truck performance in the traffic stream. Truck characteristics that affect their performance in the traffic stream are often not directly related to the bases for most-current truck classification schemes. This section briefly reviews those characteristics that are most relevant to truck performance, and discusses how they might be related to existing classification schemes. • Automated truck counting and classification considerations. Automated truck counting and classification data are becoming increasingly more available through technologies such as weigh-in-motion (WIM) and increased sampling coverage for Highway Performance Moni- toring Systems (HPMS). This section discusses those classifications and their relevance to the needs of NCFRP Project 41. • Perspectives of the trucking industry. Freight haulers and their customers have somewhat different perspectives on relevance of highway performance characteristics to their needs. For example, reliability is highly valued where shipping is for just-in-time inventories. • Recommended HCM classification scheme. This section provides the thinking of the research team on how to adapt existing classification schemes to meet the needs of NCFRP Project 41 and, ultimately, the HCM. This thinking was tested with available data as the research progresses. S e c t i o n 5 Recommended HCM Truck Classification Scheme 4 The value of time for the load carried, facility geometry, and traffic congestion on the facility are also critical factors, but these are not characteristics of the truck itself, per se.

Recommended HcM truck classification Scheme 47 5.1 Existing National Truck Classification Schemes There are four basic national truck classification schemes and several variations on these schemes (see Exhibit 18). These four major classification schemes differ according to their purpose: 1. The national NHTSA truck classification scheme is based on gross vehicle weight rating. 2. The FHWA truck classification scheme is based on the number of axles, their configuration (tandem or single), and the number of trailers. It is designed to facilitate truck classification at weigh stations. The number of axles (in combination with loaded weight) is useful for pave- ment design. 3. The STAA (Surface Transportation Assistance Act of 1982) truck classification scheme identifies a new category of trucks eligible to operate on the National Network. However, they are subject to local controls and prohibitions when off of the National Network. These trucks are generally exceptionally long, so the STAA scheme is primarily based on length. The lengths determine the turning radii for the truck-trailer combinations. 4. The AASHTO truck classification scheme is based on the wheel base, the distance between the front wheels or trailer king pin, and the centerline of the rear wheels. This information determines the turning and tracking radii of the truck and trailer. This scheme is used for the design of islands, medians, lane widths, and turning radii on highways. The HCM currently divides heavy vehicles into three classes for capacity analysis purposes: bus, truck, and recreational vehicle (RV). These classes are used to identify the appropriate pas- senger car equivalent (PCE) for each heavy-vehicle type and to compute the amount of roadway capacity consumed by each type. The PCEs may vary by terrain and road grade. Other classification schemes are those that do not fit into one of the above categories. These schemes can include overall length classifications and those by weight such as the one used by the California Energy Commission. Most of these schemes were designed to serve purposes other than determining effects of trucks on traffic flow. The current HCM 2010 truck classification scheme was developed explic- itly for capacity analysis, but it employs a gross simplification of the varying types of trucks and their different operating characteristics. The simplification was designed to reduce data collec- tion needs when determining capacity. Existing truck classifications Length Weight Number of axles etc. Classification by determinants of traffic flow Length Power to weight Percent load distribution etc. Relationships between: existing classifications determinants • • • • • • • • • • of traffic flow Exhibit 17. Transforming existing classifications to flow-determining classifications. Classification scheme Basis No. types Purpose NHTSA Gross vehicle weight rating 8 Safety assessments FHWA Axles Trailers 13 Truck classification at weigh stations Pavement design STAA Length 2 Limit access to certain road types AASHTO Wheelbase Kingpin – rear wheel C/L 7 Geometric design HCM Heavy vehicle that is not a bus or a recreational vehicle 1 Capacity analysis Exhibit 18. National truck classification schemes.

48 Incorporating Truck Analysis into the Highway Capacity Manual 5.1.1 NHTSA Classification Scheme Vehicle manufacturers are required by federal regulations (Title 49 Code of Federal Regu- lations, Chapter V, Section 565.6) to submit information on the gross vehicle weight rating (GVWR) of the vehicle to NHTSA (see Exhibit 19). This rating may also be included with the vehicle’s vehicle identification number (VIN). The GVWR is defined as the unloaded vehicle weight plus its maximum safe load. 5.1.2 FHWA Vehicle Classification Scheme The FHWA vehicle classification scheme groups vehicles into 13 categories based on the total number of axles and the number of trailers (see Exhibit 20). A survey of state DOTs by Benekohal and Girianna (2003) reported that state DOTs generally follow the FHWA classification scheme, with some variations: • Colorado DOT condenses the FHWA classification scheme into three categories. FHWA Classes 1–3 are grouped as passenger vehicles, Classes 4–7 are grouped as single-unit trucks, and FHWA Classes 8–13 are grouped as combination trucks. • Illinois DOT classifies vehicles according to length of vehicles. Passenger vehicles are up to 21 ft. long, single units are between 22 and 44 ft., and multi-units are vehicles longer than 40 ft. • Caltrans uses FHWA classification with little modification. Caltrans added two vehicles clas- sifications in the FHWA vehicle classification system for a total of 15. FHWA Class 9 was split into two classes, single and multi-trailer. An “unclassifiable” class was added to account for an unrecognizable vehicle or equipment malfunction. • Texas DOT modifies the FHWA scheme based on the estimated range of loaded weights and horsepower for each truck classification category. An example of this is shown in Exhibit 21. 5.1.3 STAA Classification The Surface Transportation Assistance Act (STAA) of 1982 regulated the length of com- mercial motor vehicles. Congress established minimum length standards for most commercial Exhibit 19. NHTSA vehicle classification scheme. Vehicle Class GVWR Class A Class B Class C Class D Class E Class F Class G Class H Class 3 Class 4 Class 5 Class 6 Class 7 Class 8 Not greater than 1360 kg. (3,000 lbs.) Greater than 1360 kg. to 1814 kg. (3,001–4,000 lbs.) Greater than 1814 kg. to 2268 kg. (4,001–5,000 lbs.) Greater than 2268 kg. to 2722 kg. (5,001–6,000 lbs.) Greater than 2722 kg. to 3175 kg. (6,001–7,000 lbs.) Greater than 3175 kg. to 3629 kg. (7,001–8,000 lbs.) Greater than 3629 kg. to 4082 kg. (8,001–9,000 lbs.) Greater than 4082 kg. to 4536 kg. (9,001–10,000 lbs.) Greater than 4536 kg. to 6350 kg. (10,001–14,000 lbs.) Greater than 6350 kg. to 7257 kg. (14,001–16,000 lbs.) Greater than 7257 kg. to 8845 kg. (16,001–19,500 lbs.) Greater than 8845 kg. to 11793 kg. (19,501–26,000 lbs.) Greater than 11793 kg. to 14968 kg. (26,001–33,000 lbs.) Greater than 14968 kg. (33,001 lbs. and heavier) Source: Title 49 CFR, Chapter V, Section 565.6, Table II.

Recommended HCM Truck Classification Scheme 49 Class Illustration Description 1 Motorcycles: All two or three-wheeled motorized vehicles. 2 Passenger Cars: All sedans, coupes, and station wagons manufactured primarily for the purpose of carrying passengers and including those passenger cars pulling recreational or other light trailers. 3 Other Two-Axle, Four-Tire Single Unit Vehicles: All two-axle, four-tire vehicles other than passenger cars. Generally pick-up trucks, sports utility vehicles, vans. 4 Buses: All vehicles manufactured as traditional passenger- carrying buses with two axles and six tires or three or more axles. Excludes modified buses no longer capable of mass passenger transport. 5 Two-Axle, Six-Tire, Single-Unit Trucks: All vehicles on a single frame including trucks, camping and recreational vehicles, motor homes, etc., with two axles and dual rear wheels. 6 Three-Axle Single-Unit Trucks: All vehicles on a single frame including trucks, camping and recreational vehicles, motor homes, etc., with three axles. 7 Four or More Axle Single-Unit Trucks: All trucks on a single frame with four or more axles. 8 Four or Fewer Axle Single-Trailer Trucks: All vehicles with four or fewer axles consisting of two units, one of which is a tractor or straight truck power unit. 9 Five-Axle Single-Trailer Trucks: All five-axle vehicles consisting of two units, one of which is a tractor or straight truck power unit. 10 Six or More Axle Single-Trailer Trucks: All vehicles with six or more axles consisting of two units, one of which is a tractor or straight truck power unit. 11 Five or Fewer Axle Multi-Trailer Trucks: All vehicles with five or fewer axles consisting of three or more units, one of which is a tractor or straight truck power unit. 12 Six-Axle Multi-Trailer Trucks: All six-axle vehicles consisting of three or more units, one of which is a tractor or straight truck power unit. 13 Seven or More Axle Multi-Trailer Trucks: All vehicles with seven or more axles consisting of three or more units, one of which is a tractor or straight truck power unit. Adapted from FHWA, 2001 and Maryland SHA, 2012. Exhibit 20. FHWA vehicle classification scheme. truck tractor-semitrailers and for twin trailers pulled behind a truck tractor. The STAA act of 1982 changed the allowable width of commercial vehicles to 102 in. Previously, the Federal-Aid Highway Act of 1956 provided a maximum vehicle width of 96 in. (FHWA, 2004). Additionally, the STAA authorized the establishment of a “National Network,” which is a network of federal highways that includes primary Interstates where federal width and length limits for heavy vehicles would apply. The National Network contains over 200,000 miles of highways across the nation (see Exhibit 22). These STAA federal length limits are minimums that states must allow for vehicles on the National Network and for the reasonable access routes to the network (FHWA, 2004).

50 Incorporating Truck Analysis into the Highway Capacity Manual Class Weight (pounds) Power (hp) Texas FHWA Minimum Maximum 5 6 15,000 46,000 220 6 7 20,000 53,000 250 7 8 25,000 52,000 250 8 8 28,000 66,000 310 9 9 30,000 80,000 380 10 10 32,000 87,000 410 11 11 35,000 92,000 440 12 12 35,000 106,000 500 13 13 35,000 120,000 570 Note: In this exhibit, trucks are defined as vehicles with three or more axles (Middleton, 2006). Exhibit 21. Texas truck classification scheme. Exhibit 22. National Network and National Highway System (FHWA, 2009).

Recommended HcM truck classification Scheme 51 Truck Tractor-Semitrailer (Single-Trailer) Combinations The minimum length set for the semitrailer in a single-trailer combination is 48 ft., which can be higher depending on the grandfathered limit for a particular state. In a state, semitrailers up to the maximum length that were lawfully operating in a truck tractor-semitrailer combination on December 1, 1982, may continue to operate after this date. The grandfathered semitrailer lengths vary with states—for example, Florida, Minnesota, and Idaho used 48 ft.; Oregon and Pennsyl- vania used 53 ft.; Texas used 59 ft.; and Louisiana used as high as 59.5 ft. States may not impose an overall vehicle length on a truck tractor-semitrailer combination operating on the National Network even if the length of vehicle exceeds the limit imposed by federal law (FHWA, 2004). Truck Tractor-Semitrailer-Trailer (Double-Trailer) Combinations The minimum length set for trailer and semitrailer combinations on the National Network is 28 ft. States must allow use of semitrailers of 28.5 ft. in length that were in use on December 1, 1982, provided that the overall length of the combination does not exceed 65 ft. The Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) determined the maximum overall length of cargo-carrying units that states may allow for twin-trailer combinations when one trailing unit is longer than 28.5 ft. (FHWA, 2004). Trucks or Straight Trucks Trucks or straight trucks are non-articulated self-propelled cargo-carrying commercial motor vehicles. These types of trucks are subject to federal weight requirements on the Interstate system and federal width requirements on the National Network. They are not subject to federal length requirements, but would be subject to state length requirements. The maximum width limit for commercial motor vehicles on the National Network and rea- sonable access route was established to be 102 in. except in Hawaii (where it is 108 in.). The federal width limits do not apply for special mobile equipment such as the following: military or farm equipment, instruments of husbandry, road construction or maintenance machinery, and emergency apparatus including police and fire emergency equipment (FHWA, 2004). There are also specific federal length limits and provisions for six types of specialized equip- ment: automobile and boat transporter combinations, B-train combinations, beverage semi- trailers, maxi-cube vehicles, saddle mount combinations, and dromedaries. 5.1.4 AASHTO Truck Classification Scheme AASHTO classified four types of design vehicles: passenger cars, buses, trucks, and RVs. Under the truck category, the seven types of truck design vehicles are listed in Exhibit 23. The purpose of Design Vehicle Symbol Overall Length (ft.) Intermediate Semitrailer WB-40 45.5 Intermediate Semitrailer WB-50 55.0 Interstate Semitrailer WB-62 68.5 Interstate Semitrailer WB-65 or WB-67 73.5 Double Bottom–Semitrailer/ Trailer WB-67D 73.3 Triple Semitrailer/ Trailer WB-100T 104.8 Turnpike Double–Semitrailer/ Trailer WB-109D 114.0 Exhibit 23. AASHTO truck classification (design vehicles) (AASHTO, 2011).

52 incorporating truck Analysis into the Highway capacity Manual the AASHTO design vehicles is to ensure adequate lane widths, turning radii, and other geometric features for trucks in the design of streets and highways. 5.2 Other Classification Schemes Besides axles, the number of trailers, weight, and trailer length, there are other classification schemes based on overall length (facilitates automated counts), emissions (for air-quality analy- sis), and fuel consumption. 5.2.1 Length-Based Classification It is not always possible to place axle sensors to collect the 13 FHWA vehicle categories; however, it is possible to use two inductance loops or magnetic units to differentiate vehicles by total length in most cases. FHWA recommends four vehicle classes when collecting data in this fashion (FHWA, 2001). These four classes are cars (and pick-up trucks), single-unit trucks, single-trailer combination trucks, and multi-trailer trucks. The latter three vehicle types are classified as trucks based on vehicle length. In some states, the multi-trailer truck category may be unnecessary because of fewer vehicles in this category and because this category can be combined with single-trailer combination trucks. Vehicle lengths tend to vary from state to state, but the vehicle length classification that best appears to cover combined data from all states is shown in Exhibit 24. Many states can improve these results by using different length spacing boundaries to account for characteristics of their own truck fleets. However, it should be noted that total vehicle length is not a consistent indica- tor of vehicle class. 5.2.2 Energy- and Emissions-Based Truck Classifications The California Energy Commission and the California Air Resources Board have defined truck classification schemes for the respective purposes of estimating fuel use and emissions. These classification schemes are shown in Exhibit 25. Primary Description Minimum Length (>) (ft.) Maximum Length (≤) (ft.) Passenger Vehicles 0 13 Single-Unit Trucks 13 35 Combination of Trucks 35 61 Multi-trailer Trucks 61 120 Exhibit 24. Length-based classification boundaries (FHWA, 2001). California Energy Commission California Air Resources Board Class Weight (lbs) Class Weight (lbs) 1 0 – 6,000 T1 LDT 1 0 – 3,750 T2 LDT 2 3,751 – 5,750 2A 6,001 – 8,500 T3 MDT 5,751 – 8,500 2B 8,501 – 10,000 T4 LHDT1 8,501 – 10,000 3 10,001 – 14,000 T5 LHDT2 10,001 – 14,000 4 14,001 – 16,000 T6 MHDT 14,001 – 33,000 5 16,001 – 19,500 6 19,500 – 26,000 7 26,000 – 33,000 8 33,001 + T7 HHDT 33,001 + Source: Dowling Associates, 2012. Exhibit 25. Example fuel- and emissions-based classifications.

Recommended HcM truck classification Scheme 53 5.3 HCM Vehicle Classification HCM classifications were developed to assess the effects of trucks on highway capacity. The HCM defines three non-automobile vehicle types: transit buses, RVs, and trucks. These three types are grouped in the HCM under the broader category of heavy vehicles (TRB, 2010). A heavy vehicle is defined in the HCM as “A vehicle with more than four wheels touching the pavement during normal operation.” Buses, RVs, and trucks are then heavy vehicles with the following special characteristics: • A bus is defined as “A self-propelled, rubber-tired road vehicle designed to carry a substantial number of passengers (at least 16) and commonly operated on streets and highways” (equiva- lent to FHWA Class 4). • An RV is defined as “A heavy vehicle, generally operated by a private motorist, for trans- porting recreational equipment or facilities; examples include campers, motor homes, and vehicles towing boat trailers” (generally equivalent to FHWA Class 5). • A truck is defined as “A heavy vehicle engaged primarily in the transport of goods and materi- als or used in the delivery of services other than public transportation” (equivalent to FHWA Classes 5–13). Each heavy-vehicle type can be assigned its own PCE value for the purposes of capacity and operational analyses. However, the HCM groups them together for most analyses. Some exam- ples of how the HCM groups heavy vehicles for different facilities include • Freeways and multilane highways—the HCM groups buses and trucks together, assigning them the same PCE values. RVs are given a slightly lower PCE value. • Two-lane highways—the HCM does not provide a PCE for buses. Trucks are assigned a PCE, and RVs are given a slightly lower PCE value. • Urban streets—a single PCE value of 2.0 is used for the entire group of heavy vehicles. HCM 2010 has adopted a capacity estimation method using PCEs that were calibrated for a mix of trucks and buses in an average weight-to-horsepower ratio of between 125 and 150 lb/hp. (HCM 2010: Chapter 11). However, Middleton (2006) found a variety of trucks with higher weight-to-power ratios. Middleton’s research indicated that the weight-to-horsepower values could reach 210 lb/hp. This is the basis of the tabulated maximum weights by FHWA weight class shown in Exhibit 26. As can be seen in Exhibit 26, the HCM heavy-vehicle classifications are much broader than other national classification schemes. They do not take into account that truck performance characteristics are determined to a large degree by weight-to-power ratio. Historically, this Class Weight (pounds) Power (hp) **Range of Weight/Horsepower FHWA Minimum Maximum 6 15,000 46,000 220 68 – 209 7 20,000 53,000 250 80 – 212 8 28,000 66,000 310 90 – 213 9 30,000 80,000 380 79 – 211 10 30,000 87,000 410 73 – 212 11 35,000 92,000 440 80 – 209 12 35,000 106,000 500 70 – 212 13 35,000 120,000 570 61 211 *Researchers defined trucks as vehicles with three or more axles (Middleton, 2006). **Estimated from weight and power values. Exhibit 26. Physical and performance characteristics of trucks.*

54 incorporating truck Analysis into the Highway capacity Manual simplistic approach was taken by the HCM to reduce data collection requirements for capacity analysis. However, with the new automated data collection sources available (specifically Weigh- in-Motion), it may now be possible to use more disaggregate classifications of heavy vehicles in an HCM analysis to yield more accurate performance and capacity estimates. 5.4 Determinants of Truck Performance in a Traffic Stream The following are the main factors that determine effects of truck performance on traffic flow: • Engine power (net power delivered to the drive axle), • Gross vehicle weight, • Losses due to rolling resistance and aerodynamic drag, • Terrain type (percent grade and length of grade), and • Vehicle length. The first four of these determine the maximum acceleration rate for a truck and how much a truck will slow down on a grade. The fifth is the physical space taken up by a truck. Although it is often assumed that vehicle length represents the amount of capacity taken up by a truck, investigations have shown that effects of trucks on traffic flow are far more complex. Traffic flow shows dependence on the mix of trucks, overall truck percentage, and grade (Rakha et al., 2007). Investigation of truck acceleration characteristics shows a critical dependence of maxi- mum acceleration rates to power-to-weight ratios (Rakha et al., 2001; Middleton, 2006). On level terrain, it can be shown that the maximum acceleration of a vehicle is given by 8226 Equation 6maxa WEHPR V( )= p where amax = maximum acceleration (mph/s), WEHPR = weight-to-effective horsepower ratio (lb/hp), and V = vehicle speed (mph). The effective horsepower is what is available for acceleration after the effects of transmission loss, aerodynamic drag, and rolling resistance are accounted for. Therefore, for a truck with a WEHPR of 300 traveling at 55 mph, the maximum acceleration would be about 0.5 mph/s. Middleton (2006) showed how truck acceleration characteristics were determined for various vehicle classes, which were used to calibrate a traffic microsimulation model. Weight-to-horsepower ratio is also a determinant of the maximum sustainable speed of a vehicle on a grade. The maximum sustainable speed of a truck on a grade is given by 375 Equation 7maxV WEHPR g( )= p where Vmax = the maximum sustainable speed, g = grade (expressed as a fraction), and WEHPR = weight-to-effective horsepower ratio (lb/hp). For example, a truck with an effective WEHPR of 300 on a grade of 6% can sustain a maxi- mum speed of only about 20 mph. Trucks entering a steep grade typically have their speeds “decay” to this maximum sustainable speed within about one-quarter to one-half mile.

Recommended HcM truck classification Scheme 55 Weight-to-horsepower also determines how quickly a vehicle can accelerate from a standing start. This is an important factor when trucks operate on signalized roadways (arterials). The minimum time for a vehicle at a standstill with a fixed WEHPR to accelerate to a speed V on a level surface is given by 6.1 10 Equation 8min 5 2t WEHPR V= −p p p where V = speed (mph), tmin = minimum time (s), and WEHPR = weight-to-effective horsepower ratio (lb/hp). A truck with a WEHPR of 300 would take a minimum of 16 seconds to reach a speed of 30 mph, for example. Given the dependence of truck performance on weight-to-horsepower ratio, it can be seen from the weight and horsepower ranges shown in Exhibit 26 that the FHWA truck classification scheme covers a very large range of actual truck performance capabilities. For urban streets and intersections, there are issues with low saturation flow rates for trucks at signals; low progression speeds on arterials; and gap acceptance at unsignalized intersections and roundabouts. 5.5 Data Collection Considerations A classification scheme that requires significant effort to collect the data needed for analysis will reduce the likelihood of agencies using any classification scheme developed. Recent advances in automated data collection technologies are making it easier to collect large amounts of truck data with less effort on the part of transportation agencies. These automated systems are able to collect various truck characteristics that have expanded the possibilities for employing more elaborate truck classification schemes than are currently used in the HCM. There are different automated data collection technologies and each technology has its strengths and weaknesses. The cost, reliability, precision, life span, installation, maintenance, and type of data provided also vary with the technology (Benekohal and Girianna, 2003). Vehicle classification technologies can be grouped into the following three categories: axle- based, vehicle-length-based, and machine-vision-based. Benekohal and Girianna suggested that the accuracy of the classifiers depends on several factors including the type of sensor used (loop, tube, piezoelectric, etc.); roadway geometry conditions at the site of classification; installation and maintenance; and classification algorithms. Some errors are due to incorrect measurement of the number of axles, a considerable change in vehicle speed over the sensors, or vehicles that do not fit into any of the defined classes of vehicles. 5.5.1 Video Camera Data Collection Video cameras with the appropriate software can classify vehicles based on overall length. A single sensor or combinations of different types of sensors are used. Based on this detection method, there may be fewer categories than FHWA’s 13 vehicle classes because of the difficulty of differentiating a single long vehicle unit from two smaller or shorter units hitched together. The vehicle length classifiers remain popular in some states because fewer categories are suffi- cient for a variety of traffic monitoring purposes (Benekohal and Girianna, 2003). The machine vision–based classifiers combine video imaging with computerized pattern rec- ognition. A video camera is used to record video images that are taken continuously at regular

56 incorporating truck Analysis into the Highway capacity Manual time intervals. A digitizer converts the frames into digital signals that are sent to a computer for extraction of vehicle features. Some of the limitations of the machine vision–based classifier are measuring speed accurately and difficulties in differentiating among closely spaced vehicles. Research and development are underway on new sensor technologies to obtain more accurate vehicle classifications. 5.5.2 Weigh-In-Motion Data Collection WIM devices measure axle weights and gross vehicle weights as vehicles drive over the detec- tion site. They are more efficient than traditional weigh stations because they can measure weights at normal operating speeds. WIM data provides information on the distribution of actual truck weights as they are measured at the time. It can provide a finer stratification of trucks by weight than can be provided using just the FHWA vehicle classification scheme. Data are also provided on truck speeds, although these are usually “binned” into 5 mph groups. 5.5.3 Highway Performance Monitoring Systems Truck Classification Data HPMS provide data on pavement conditions, geometry, terrain, weather, and traffic counts on a selected sample of roads across various functional classes. A subset of the sample data contains information on percentage truck traffic by two classes: single-unit trucks and combination-unit trucks. This data may be estimated or it may be counted. HPMS data are in some sense broader in coverage than most other traffic classification data sources. HPMS truck categories are often too broad to be of much use for operational analysis. Counts are presented as average daily values and average percent trucks, which are not useful for analyses of peak periods of congestion. Agencies must have the labor and analytical capacity required to process the HPMS data collected. 5.6 Trucking/Shipping Industry Perspectives From the point of view of shippers and carriers, the value of time for a given truck depends more on the type of load carried than on the weight of the load. A truck hauling gravel will have a different value of time (and value of reliability) than a concrete truck hauling wet concrete or high value, just-in-time delivery, electronic goods. The available data and the available classifica- tion schemes do not provide a means to sort low time value loads from high time value loads. 5.7 Recommended NCFRP Project 41 HCM Truck Classification Scheme The truck classification scheme for purposes of this research should serve the following purposes: • Accurately determine the effects of trucks on traffic operations according to the mix of trucks of different types in the traffic stream and • Provide managers and decisionmakers with sufficient information to estimate the cost- effectiveness of different types and scales of operational improvements and capacity enhancements. It is clear that the available truck classification schemes do not directly serve these purposes. They are either directed at truck characteristics that bear only indirectly on these purposes, or they are so broad that they mask truck performance differences that affect traffic flow and, there- fore, lose accuracy in the process.

Recommended HcM truck classification Scheme 57 The ideal truck classification scheme would take into account • Truck length and weight-to-horsepower ratio (so as to estimate the capacity and speed effects of trucks on automobile traffic) and • Whether the truck is loaded or unloaded and the value of the goods carried (so as to estimate the importance of reliability and average travel time to trucks). At this point in time, it is not practical to gather data on these characteristics of trucks using any given facility. The FHWA vehicle classification scheme—although it lacks the ability to take into account weight-to-horsepower ratio—has the advantage of being a nationally established classification scheme for which technologies have been developed for the automatic collection of data by vehicle class. The FHWA vehicle classification scheme is based primarily on the number of axles, which is a rough proxy for vehicle length. WIM stations are capable of assigning measured weights by vehicle class at the station. Given the lack of information on specific weight-to-horsepower ratios by vehicle class, it is recommended that the number of FHWA vehicle classes be condensed from 13 to 5 for highway capacity analysis purposes: 1. Motorcycles (FHWA Class 1); 2. Passenger vehicles (FHWA Classes 2, 3); 3. Buses (FHWA Class 4); 4. Single-unit trucks (FHWA Classes 5–7); and 5. Semi-trailer combination trucks (FHWA Classes 8–13). The recommendation to split trucks into two types (single-unit and semitrailer trucks) is supported by statewide data on truck types collected by Stone et al. (2010) for more than 600 locations where vehicle classification counts were collected as well as more than 50 WIM stations. Overwhelmingly, the trucks fell into FHWA Truck Classes 5 and 9. The following table from Stone et al. (see Exhibit 27) shows the breakdown for 10 random WIM locations. Note the high percentage of Class 5 and 9 trucks. These two classes alone account for 75% of the truck traffic observed on North Carolina roads. ID Route Date VC4 VC5 VC6 VC7 VC8 VC9 VC10 VC11 VC12 VC13 Total VC1903 US 64 02-27-06 5% 31% 8% 0% 16% 37% 3% 0% 0% 0% 100% VC1904 NC 294 02-20-07 4% 44% 11% 0% 15% 26% 0% 0% 0% 0% 100% VC1905 NC 60 02-20-07 4% 39% 9% 1% 6% 33% 7% 0% 0% 1% 100% VC1902 US 19 11-13-06 5% 47% 11% 0% 7% 27% 2% 0% 0% 1% 100% VC2104 US 64 05-01-07 3% 49% 9% 0% 9% 28% 2% 0% 0% 0% 100% VC2102 NC 69 09-25-06 4% 23% 5% 1% 10% 56% 1% 0% 0% 0% 100% VC2103 NC 175 02-20-07 6% 66% 11% 1% 10% 5% 0% 0% 0% 1% 100% VC5508 US 64 05-01-07 4% 16% 5% 0% 5% 65% 1% 3% 1% 0% 100% VC5501 US 23 10-09-06 6% 19% 2% 0% 8% 61% 1% 2% 1% 0% 100% VC3701 US 129 08-22-06 3% 20% 5% 1% 8% 59% 3% 1% 0% 0% 100% Average 4% 35% 8% 0% 9% 40% 2% 1% 0% 0% 100% VC = FHWA Vehicle Class; adapted from Stone et al., 2010. Exhibit 27. Percentage of trucks on road by FHWA class—North Carolina.

58 incorporating truck Analysis into the Highway capacity Manual Future research may be able to develop data collection methods so that analysts conducting capacity analyses can segregate the single unit trucks and the semitrailer trucks observed on a given facility by their weight-to-horsepower ratio, thereby further refining their ability to esti- mate the capacity and speed effects of trucks. Note that motorcycles, passenger vehicles, and buses are not the subject of the current research project. The HCM already includes methods for evaluating the capacity effects of passenger vehicles and buses. Also note that RVs, currently a separate non-truck vehicle type in the HCM, fall within the FHWA definition for Vehicle Class 5.

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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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