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5 2.1 Introduction Trucks are an important consideration in the design and access management of road- ways. The size, weight, height, and operational characteristics of trucks are uniquely different from that of passenger cars and need to be a factor when designing geometric elements of the roadway and selecting access management treatments for the roadway. Physical character- istics of trucks (e.g., length, width) and operational characteristics and needs of trucks (e.g., turning path, acceleration and deceleration, parking needs) are crucial to assessing the traffic operational and safety performance of a roadway, particularly when trucks constitute a signifi- cant portion of the vehicle mix. 2.2 Trucks as a Design Vehicle Trucks are wider and longer than buses and passenger cars, and therefore the geometric design requirements for trucks are greater than for other design vehicles. In selecting an appropriate design vehicle for any roadway, the designer should consider the largest design vehicle that is likely to use that roadway with considerable frequency, or a design vehicle with special charac- teristics appropriate to a particular location (AASHTO 2018). The design vehicle is used in the design of critical features such as curb return radii, turning radii at intersections, and driveway width, which are typically designed to accommodate the turning radius and swept path width of a specific design vehicle. The design vehicle is also used in such roadway features as storage length of turn lanes and driveway throat length, which are typically designed to accommodate the length of a specific design vehicle. Section 4.2 presents a detailed discussion of design vehicles and considerations for selecting an appropriate design vehicle. 2.3 Turning Characteristics of Trucks Turning maneuvers are one of the key challenges faced by trucks on urban and suburban arterials. Because of their larger width and length, trucks require a larger turning path than other vehicles for making a turning maneuver. Specifically, trucks exhibit offtracking, which is a function of the spacing between the truckâs tire axles. A turning template for a specific design vehicle, or an automated version of a turning template in computer-aided design and drafting (CADD) software, can be used to assess whether truck encroachment on the curbline or another lane will occur. Some transportation agencies have indicated that right turns at intersections are the most problematic for trucks. Where trucks make right turns at intersections, it is desirable to provide a sufficiently large curb return radius so that a truck can make a right turn without encroaching C H A P T E R 2 Overview of Truck Operational Challenges and Needs
6 Design and Access Management Guidelines for Truck Routes: Planning and Design Guide on the curbline or on an adjacent or opposing lane. However, when balancing the needs of trucks with other user modes, a transportation agency may (if the applicable state vehicle code allows) find it desirable to accept occasional encroachment on adjacent or opposite lanes. Left-turn maneuvers at intersections may be less critical for trucks than right-turn maneuvers, but left turns can still present challenges for trucks. Double left-turn lanes are implemented by some transportation agencies to accommodate trucks because the outer (rightmost) left-turn lane can provide a larger turning radius. Section 4.3 discusses truck-turning considerations in the design of intersections. The turning characteristics of trucks are also an important consideration in the design of inter- change ramp terminals. It is important to provide sufficient turning radii for trucks turning from the ramp to the crossroad and for trucks turning from the crossroad to the ramp. Section 4.4 presents a discussion of the following five types of interchange designs, and how trucks can be considered in each design: ⢠Conventional diamond interchanges and closely related interchange configurations. ⢠Single-point diamond interchanges. ⢠Full cloverleaf interchanges. ⢠Diverging diamond interchanges. ⢠Roundabout interchanges. Where trucks are expected to use a driveway, particular consideration should be given to the turning capabilities of the design vehicle when designing the driveway. In the selection of curb return radius and throat width, and in the decision whether or not to include a median in the driveway, designers should consider the larger turning radii that trucks need when making turns into and out of a driveway (see Section 4.5). In higher-speed environments, particularly in rural areas, large trucks may experience off- tracking when negotiating horizontal curves. To address this issue, the traveled way may need to be widened at some horizontal curves to fully accommodate trucks (see Section 4.6.2). 2.4 Storage Length for Trucks The length of trucks is a key consideration when selecting an appropriate storage length for turn lanes, intersection spacing within an interchange, and the throat length of a driveway. A common design vehicle on truck routes and freight corridors is the WB-67, which is 73.5 ft in length. In comparison, a single-unit truck is approximately 30 ft in length and a passenger car is approximately 20 ft in length. If left- or right-turn lanes are not designed with sufficient storage length, the queue of vehicles waiting to turn may extend into the adjacent lane and block through-moving vehicles. Section 4.3.3 presents queue storage length adjustments for trucks, which can be considered in determining an appropriate length for a left-turn lane to accom- modate trucks. Storage length is also an important consideration in interchange design. Sufficient spacing between the crossroad ramp terminals is desirable to provide sufficient storage space for large vehicles within the interchange on the arterial crossroad. Storage space at interchanges is discussed further in Section 4.4. Where trucks are expected to use a driveway, particular consideration should be given to the length of the design vehicle when designing the driveway. The determination of an appropriate driveway throat length should be based on the length of the design vehicle. The driveway throat needs to be long enough for trucks to enter or exit the site without interfering with onsite traffic or with through traffic on the roadway. Section 4.5 presents a discussion of the consideration of trucks in driveway design.
Overview of Truck Operational Challenges and Needs 7 2.5 Truck Acceleration and Deceleration Trucks have quite different operating characteristics than passenger cars when accelerating from a stopped position or decelerating to a stopped position. This is an important distinction when considering the benefits of installing a left- or right-turn lane. A truck decelerates and accelerates more slowly than a passenger car; therefore, when a truck has to stop behind a turning vehicle, it causes additional delay to the vehicles following the truck. A turn lane removes the turning vehicles from the through lane and minimizes the deceleration and acceleration of all traffic, including trucks. For example, because of the slower acceleration of trucks, drivers of trucks may be more selective in choosing a gap when making a left-turn maneuver. A left-turn lane provides a sheltered location for drivers to wait for a gap in opposing traffic. Sections 4.3.3 and 4.3.4 present a more detailed discussion of the consideration of trucks in the need for and design of left- and right-turn lanes, respectively. 2.6 Truck Deliveries and Parking Businesses typically generate demand for pickups and deliveries whether loading zones are provided are not. Furthermore, in recent years the demand for truck deliveries has increased substantially with the increased popularity of e-commerce. Given the desire by transportation agencies to serve all travel modes effectively, there are increasing demands for space in the road- way cross section, which may limit the space available for trucks to stop and make pickups and deliveries. Trucks must often stop along roads and streets to make pickups and deliveries at locations where off-road loading and unloading areas are not available, and where no curbside loading zones are provided. This is an ongoing challenge for trucking companies, who risk a citation for parking in a center two-way left-turn lane, through lane, or bike lane to complete their delivery or pickup. Section 3.1.5 presents a further discussion of challenges related to truck deliveries. There is a nationwide shortage of truck parking places, either to accommodate a driverâs need for rest or for drivers to wait for their scheduled delivery time (due to early arrival). The shortage of parking areas has existed for a long time, but the lack of parking areas has recently become a more critical issue with the new federal mandate for on-vehicle electronic data loggers and the increased freight demand due to the increase in e-commerce. Many more drivers are now choosing to park their trucks illegally, in exposed positions on highway shoulders or on ramps. Section 3.7 presents a more detailed discussion of challenges related to truck parking. 2.7 Balancing Truck Needs with Other Modes While truck routes should be designed to accommodate the types of trucks that use the route with considerable frequency, the needs of other modes present on the truck routeâsuch as pedestrians, bicyclists, and transitâshould be considered as well. This does not necessarily mean that facilities for every mode are provided, but rather the appropriate balance among the needs of each mode be sought throughout the truck route. Chapter 5 summarizes the key issues in balancing truck considerations with the needs of pedestrians, bicyclists, and transit.
