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1 This guide summarizes available knowledge on the interactions of more than 70 access management techniques with the operations and safety of motorized vehicles, pedestrians, bicyclists, buses, and trucks. Where they are available, quantitative tools are identified and described that can be used to evaluate the magnitude of the interaction on a specific travel mode. In other cases, no quantitative tools exist, but qualitative relationships are documented in the literature. These qualitative relationships are also summarized in the guide. Finally, no research has been performed on the interaction between a specific access management technique and a specific mode. This lack of knowledge is also documented in the guide and can serve as a starting point for identifying future research needs. This guide has been written for access management practitioners who already have some familiarity with the range of potential access management techniques. The guide extends the information provided in the Access Management Manual, Second ed. (1) and the Access Management Application Guidelines (2) to present current knowledge about the multimodal effects of access management techniques. However, the guide does not intend to duplicate information found in these two basic references. To help readers learn more about how to implement specific access management techniques, each section of the guide provides cross-references to specific sections of the Access Management Manual and Access Man- agement Application Guidelines, along with other relevant reports that provide additional information. Defining Operational and Safety Performance Operational performance reflects the ease with which a traveler can move along a roadway. Safety performance reflects the chances of a traveler being involved in a crash or a close call while on the roadway. A small number of performance measures are commonly used to describe operational and safety performance; these measures are described as follows. Common Operational Performance Measures Motorized Travel Modes (Automobile, Bus, and Truck) These operational performance measures frequently describe the operations of motorized travel modes: â¢ Delay. Extra travel time required to travel along the street compared with the time required to travel at the posted or free-flow speed. Delay can be caused by waiting for a red light to change at a traffic signal, waiting for a gap in traffic to turn onto a street, and S U M M A R Y Guide for the Analysis of Multimodal Corridor Access Management
2 Guide for the Analysis of Multimodal Corridor Access Management being stuck behind a vehicle waiting to make a turn, among other causes. For buses, delay can also occur when a bus has to wait for a gap in traffic before it can proceed down the street after serving a bus stop. â¢ Travel speed. The speed that vehicles drive along the street. This speed can be expressed as an average travel speed that includes delays, or as a free-flow speed, an average midblock travel speed without delays, under low-traffic-volume conditions. â¢ Stopping rate. The average number of times per mile that a vehicle must come to a stop. This measure is particularly relevant to buses and trucks, which take more time than automobiles to accelerate back to their running speed and therefore experience more delay with every stop, compared with automobiles. â¢ Queue length. The length of a line of stopped vehicles (for example, waiting for a green light or waiting for a gap in traffic to make a turn). This length is often expressed as a 95th percentile lengthâthe maximum length observed or expected once every 20 timesâ for the purposes of sizing turn lanes and locating driveways. Queues that spill out of a turn lane into a through travel lane or that block driveways can cause both operational and safety problems. â¢ Level of service. Speed and delay can also be expressed as a level of service (LOS), a letter A (best) to F (worst) that is assigned to ranges of speeds or delays according to tables provided in the Highway Capacity Manual: A Guide for Multimodal Mobility Analysis (HCM6) (3). Improvements in motorized vehicle operational performance are characterized by reduced delay, increased travel speeds, fewer stops, shorter queues, or improved levels of service. Note that an improvement in a given modeâs operational performance is not always the desired outcome in the bigger picture; for example, faster auto travel speeds are typically associated with reduced bicycle and pedestrian operational performance (level of service) and, depending on the circumstances, may result in reduced safety performance for one or more travel modes. Non-Motorized Travel Modes (Pedestrian and Bicycle) These operational performance measures frequently describe the operations of non- motorized travel modes: â¢ Delay. Extra travel time required to travel along the street. Delay can be caused by waiting for a walk signal at a crosswalk, waiting for a gap in traffic to cross a street, or extra travel time required when forced to travel out of direction. â¢ Level of service. The HCM6 defines pedestrian and bicycle LOS as a function of a number of factors, including the quality of the pedestrian/bicycle infrastructure, motorized traffic volumes and speeds, and parking presence, among others (see the appendix for more details). LOS can be expressed as a numerical value that is the average level of satisfaction that pedestrians or bicyclists would rate the facility or intersection. These scores can also be converted into A (best) to F (worst) letters using tables in the HCM6. Improved pedestrian operational performance and bicycle operational performance are characterized by reduced delay or improved level of service. Common Safety Performance Measures These safety performance measures frequently describe the safety of various travel modes: â¢ Crash rate. The number of crashes per number of vehicles or distance traveled (e.g., crashes per million vehicles entering an intersection or crashes per million vehicle miles
Summary 3 traveled on a roadway). Because the number of crashes increases with traffic volume, crash rates are used to compare different roadways on an apples-to-apples basis, by accounting for the different traffic volumes on the two roadways. â¢ Conflict points. Conflict points are locations where the paths of two vehicles can cross. The greater the number of turning movements allowed at an intersection or driveway, the greater the opportunity for two vehicles to come in conflict with each other, and the greater the potential for drivers to make mistakes (1). â¢ Conflicts. Some types of crashes, such as those involving pedestrians and bicyclists, are relatively rare but very serious when they do occur. Because of the relatively low number of these types of crashes, it can be difficult for research to quantify the change in crash rate expected to result from applying a particular access management technique. Instead, some research has investigated the change in the number of conflicts (e.g., near misses, sudden maneuvers, or hard braking) that occur following the use of a particular technique. Improved safety performance is characterized by reduced crash rates, reduced number of conflict points, or reduced number of conflicts. Organization of the Guide Overview Each chapter addresses one of 19 groups of related access management techniques, with each group containing between one and eight techniques. Each of these techniques, with the exception of installing a roundabout in lieu of a traffic signal, has been described in NCHRP Report 420: Impacts of Access Management Techniques (4). Each chapter is organized simi- larly, with information presented in order from the most general (performance summaries) to more detailed (qualitative descriptions of performance trends) to the most detailed (descriptions of available tools for quantifying specific operations and safety interactions by mode). Finally, an appendix provides detailed guidance on applying the most common tools available for quantifying the interactions of access management techniques. Access Management Technique Groups The 19 groups of techniques covered in the guide consist of the following access manage- ment technique groups presented in consecutive chapters: Chapter 1 Restrict Left-Turn Movements at an Access Point Chapter 2 Non-Traversable Medians Chapter 3 Continuous Two-Way Left-Turn Lanes Chapter 4 Frontage and Service Roads Chapter 5 Unsignalized Median Openings Chapter 6 Traffic Signal Spacing Chapter 7 Number and Spacing of Unsignalized Access Points Chapter 8 Interchange Areas Chapter 9 Left-Turn Lanes Chapter 10 Right-Turn Lanes Chapter 11 Driveway Channelization Chapter 12 Alternative Intersections and Interchanges Chapter 13 Parking and Stopping Restrictions Chapter 14 Roundabouts Chapter 15 Driveway Sight Distance Chapter 16 One-Way Driveways
4 Guide for the Analysis of Multimodal Corridor Access Management Chapter 17 Driveway Width Chapter 18 Driveway Vertical Geometry Chapter 19 Driveway Throat Length Description Each chapter begins with a general description of the characteristics of the techniques included in that group as well as a representative photograph. Multimodal Operations and Safety Performance Summary The table below summarizes general performance trends and documented performance relationships associated with each access management technique in the group. The table is organized by technique, travel mode, and the areas of operations and safety. Hollow dots indicate combinations of techniques and travel modes for which no performance relation- ship has been documented. All other symbols indicate the existence of a relationship for that combination of technique and mode. Possible general performance trends are improved performance, decreased performance, mixed performance, unchanged performance, and no relationship documented. The mixed performance category is used when (a) a technique produces both positive and negative interactions with a particular mode or (b) a technique in some cases has no interaction and in other cases has an interaction. The unchanged performance category is used when the interaction has been studied and no change in performance was documented. Performance trends are generally associated with a suburban/urban land use context and may differ under a central business district context for certain access management techniques. The two pos- sible types of documented relationships are quantitative and qualitative. The performance trend information presented in Table 1 does not indicate the magnitude of the interaction, either by itself or relative to other techniques. In some cases, the answer depends on other factors. Consult the detailed information presented later in each chapter to make these determinations. Entries for motor vehicles (i.e., the car symbol) are based on the cited research that may be specific to only passenger cars or, more broadly, representative of all forms of motor vehicles, including trucks, buses, and so forth. To understand the specific context, readers should consult the cited research to identify the specific performance trends. Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Install continuous TWLTL. â â â â â â â â Note for multimodal operations and safety performance summary tables: = no relationship documented. General performance trends: â = improved performance, â = decreased performance, and â = mixed performance. Documented performance relationships: = quantitative relationship and = qualitative relationship. Performance trends are associated with a suburban/urban land use context and may differ under a central business district context. Entries for buses and trucks indicate relationships specific to those modes; motor vehicle relationships also apply. Table 1. Multimodal operations and safety performance summary.
Summary 5 Entries for the bus and truck modes reflect interactions specific to those modes. However, buses and trucks are also types of motor vehicles; therefore, the information presented for motor vehicles in general also applies to buses and trucks. Nevertheless, in some cases, the magnitude of a particular interaction may be different for buses and trucks from motor vehicles in general. Buses or trucks may experience additional interactions that motor vehicles do not. These differences are reflected in the entries in the bus and truck columns of the tables. General Trends Associated with Improvements Each chapter has a table that describes the documented interactions between access management techniques and a given modeâs operations and safety. Sources are also listed. Quantitative Analysis Methods Where one or more quantitative analysis methods exist for a given combination of mode and operations or safety, this subsection provides information about those methods. For relatively simple methods, such as crash modification factors, the specific relationship is presented in the text, along with a reference to the source document. For complex methods (e.g., methods found in the HCM6), this subsection provides a reference to the source document and provides guidance on the potential magnitude of the relationship. The appendix provides additional guidance on quantitative methods that appear in multiple guide sections. Additional Information This subsection provides cross-references to additional sources of information about this group of 19 access management techniques. These sources include the following: â¢ Other chapters in this guide. â¢ Specific chapters and sections within the Access Management Manual, Second ed. â¢ Specific chapters and sections within the Access Management Application Guidelines. â¢ NCHRP reports and other authoritative documents. References 1. Williams, K. M., V. G. Stover, K. K. Dixon, and P. Demosthenes. Access Management Manual, Second ed. Transportation Research Board of the National Academies, Washington, D.C., 2014. 2. Dixon, K. K., R. D. Layton, M. Butorac, P. Ryus, J. L. Gattis, L. Brown, and D. Huntington. Access Management Application Guidelines. Transportation Research Board, Washington, D.C., 2016. 3. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 4. Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board, Washington, D.C., 1999.