PART 1 Components of the Traffic System and their Characteristics
PART 1 Components of the Traffic System and their Characteris tics
CHAPTER 2 Road User and vehicle Characteristics 2.1 Overview of Traffic Stream Components f o de. eral environment also has an impact on traffic operations,but this is difficult to assess in any To begin to understand the functional and operational aspects of traffic on streets and highways it is important given situation.Such things as weather,lighting,density to understand how tem intera aicopcraioTct2Sio limitations of each of these ele suppmivenoma ical components that interact in a traffic system: in making judgments. road users drivers,pedestrias,bicyclists.and 2.1.1 Dealing with Diversity nassengers ·Vehicles -private and commercia Streets and highways arious com ystem ha Traffic control devices .The general environmer could be more easily achieved if all vehicles had uniform while with elderly drivers as well as 18-year-olds,aggressive 17
CHAPTER Road User and Vehicle Characteristics 2.1 Overview of Traffic Stream Chapter 4 provides an overview of traffic control deComponents To begin to understand the functional and operational aspects of traffic on streets and highways it is important to understand how the various elements of a traffic system interact. Further, the characteristics of traffic streams are heavily influenced by the characteristics and limitations of each of these elements. There are five critical components that interact in a traffic system: Road users-drivers, pedestrians, bicyclists, and Veh-icles-private and commercial Strelets and highways Traffic control devices The general environment pass,engers This chapter provides an overview of critical road user arid vehicle characteristics. Chapter 3 focuses on the characteristics of streets and highways, while vices and their use. The general environment also has an impact on traffic operations, but this is difficult to assess in any given situation. Such things as weather, lighting, density of development, and local enforcement policies all play a role in affecting traffic operations. These factors are most often considered qualitatively, with occasional supplemental quantitative information available to assist in making judgments. 2.1 .I Dealing with Diversity Traffic engineering would be a great deal simpler if the various components of the traffic system had uniform characteristics. Traffic controls could be easily designed if all drivers reacted to them in exactly the same way. Safety could be more easily achieved if all vehicles had uniform dimensions, weights, and operating characteristics. Drivers and other road users, however, have widely varying characteristics. The traffic engineer must deal with elderly drivers as well as 18-year-olds, aggressive 17
B CHAPTER 2 ROAD USER AND VEHICLE CHARACTERISTICS drivers and timid drivers.and drivers subject to myriac Just as road-user characteristics vary.the character distractions both inside and outside their vehicles.Sim- istics of vehicles vary widely as well.Highways must be designed to accommodate motorcycles,the full range of ad,wa ome complex because no two Most human characteristics follow the normal dis- Thus lane widths for examle must tribution (see Chapter 8).The normal distribution is date the largest vehicles expected to use the facility. Some control over the range of road-user and vehi- ve c cteristic: s on ven e design an are a few pedestrians that walk either much slower or measures.the traffcengineer must still deal with a wide much faster.A normal distribution defines the propor range of road-user and vehicle characteristics. tions of the population expected to fall ranges. for "av 2.1.2 Addressing Diversity through chara opacteaoeslg timed.for example,to accommodate the average speed Uniformity of crossing pedestrians,about half of all pedestrians While traffic engineers have little over drive dway system red to the "85th centile (characteristic.In general ractice In hoth case of uniformity of terms.a percentile is a value in a distribution for which approach is desirable.Roadways of a similar type and function should have a familiar"look"to drivers,traffic contro devices should be asuniform as possible 1 。a5hpe form ways While this does not assure uniform reactions centile walking speed is used,as only 15%of the popu nsnamm tile yalee cues 5tn perce qual to than this alu This roach leads to trols,r design practices and procedures that safely accommo date 85%of the population.What about the remaining 2.2 Road Users and lo nd to practical terms,the highest and lowest 15%of the distri Human beings are complex and have a wide range of bution represent very extreme values that could not be effectively accommodated into design practces Qual characteristics not within the 85th (or is a keen understanding of driver characteristics Much of considered,but most standard practices and criteria do he task of traffic engineers is to find ways to provide drivers with information in a clear,effective manner that nduces sate and proper responses
18 CHAPTER 2 ROAD USER AND VEHICLE CHARACTERISTICS drivers and timid drivers, and drivers subject to myriad distractions both inside and outside their vehicles. Simple subjects like reaction time, vision characteristics, and walking speed become complex because no two road users are the same. Most human characteristics follow the normal distribution (see Chapter 8). The normal distribution is characterized by a strong central tendency (i.e., most people have characteristics falling into a definable range). For example, most pedestrians crossing a street walk at speeds between 3.0 and 5.0 ft/s. However, there are a few pedestrians that walk either much slower or much faster. A normal distribution defines the proportions of the population expected to fall into these ranges. Because of variation, it is not practical to design a system for “average” characteristics. If a signal is timed, for example, to accommodate the average speed of crossing pedestrians, about half of all pedestrians would walk at a slower rate and be exposed to unacceptable risks. Thus, most standards are geared to the “85th percentile” (or “15th percentile”) characteristic. In general terms, a percentile is a value in a distribution for which the stated percentage of the population has a characteristic that is less than or equal to the specified value. In terms of walking speed, for example, safety demands that we accommodate slower walkers. The 15th percentile walking speed is used, as only 15% of the population walks slower than this. Where driver reaction time is concerned, the 85th percentile value is used, as 85% of the population has a reaction time that is numerically equal to or less than this value. This approach leads to design practices and procedures that safely accommodate 85% of the population. What about the remaining 15%? One of the characteristics of normal distributions is that the extreme ends of the distribution (the highest and lowest 15%) extend to plus or minus infinity. In practical terms, the highest and lowest 15% of the distribution represent very extreme values that could not be effectively accommodated into design practices. Qualitatively, the existence of road users who may possess characteristics not within the 85th (or 15th) percentile is considered, but most standard practices and criteria do not directly accommodate them. Where feasible, higher percentile characteristics can be employed. Just as road-user characteristics vary, the characteristics of vehicles vary widely as well. Highways must be designed to accommodate motorcycles, the full range of automobiles, and a wide range of commercial vehicles, including double- and triple-back tractor-trailer combinations. Thus, lane widths, for example, must accommodate the largest vehicles expected to use the facility. Some control over the range of road-user and vehicle characteristics is maintained through licensing criteria and federal and state standards on vehicle design and operating characteristics. While these are important measures, the traffic engineer must still deal with a wide range of road-user and vehicle characteristics. 2.1.2 Addressing Diversity through Uniformity While traffic engineers have little control over driver and vehicle characteristics, design of roadway systems and traffic controls is in the core of their professional practice. In both cases, a strong degree of uniformity of approach is desirable. Roadways of a similar type and function should have a familiar “look” to drivers; traffic control devices should be as uniform as possible. Traffic engineers strive to provide information to drivers in uniform ways. While this does not assure uniform reactions from drivers, it at least narrows the range of behavior, as drivers become accustomed to and familiar with the cues traffic engineers design into the system. Chapters 3 and 4 will deal with roadways and controls, respectively, and will treat the issue of uniformity in greater detail. 2.2 Road Users Human beings are complex and have a wide range of characteristics that can and do influence the driving task. In a system where the driver is in complete control of vehicle operations, good traffic engineering requires a keen understanding of driver characteristics. Much of the task of traffic engineers is to find ways to provide drivers with information in a clear, effective manner that induces safe and proper responses
2.2 ROAD USERS 19 The two driver characteristics of utmost impor- tance are visual acuity factors and the reaction process moaoa0ecnGcm tion with respect to the r's eyes The two overlap.in that reaction requires the use of vi only one of thes ed issuing a driver's license.traffic engineers must expec design of roadways and controls and deal with significant variation in many of the other There are other important characteristics as well. visual characteristics of drivers. Hearing is an important element in the driving task(i.e. Fields of Vision mnetcgnranmdnolemg3 t can he desig Figure 2.1 illustrates three distinct fields of vision,each of which is important to the driving task [2]: aking systems ks other heavy vchicles Of course,one of the most important human fac- tors that influences driving is the personality and psy this fiel This,how not ca This field ma nd lef o of the 深aa的a indicated by accident and violation experience. 2.2.1 Visual Characteristics of Drivers These fields of vision stationary person.In particular.the peripheral vision When drivers initially apply for,or renew,their licenses, field narrows,as speed increases,to as little as 100at they are aslked to take an eye test,administeredeither by 20 mi/h and to 40"at 60 mi/h. The ency or by an o driver's visual landsc pe is both complex and rapidly c ects appear o motor vehicle andard relative motion both to the driver and to each other.The chart-readinge rcise that measures staric visualacuity that is.the ability to see small stationary details clearly. typical driver essentially samples the available visual in formation and selects appropriate cues to make driving Visual Factors in Driving decis The fields of vision affect a While certainly an important characteristic.static visual actices and functions Traffi acuity is hardly the only visual factor involved in the ple.are placed so that they can be read within the acute ing tas e Traffic Engin ring Handbook pro without requiring dr f visua Thu .they are generally pa Ccstochanegetherlt drivi Many of the other factors listed in Table 2.1 reflect sumed to ben ine with the highway alignment.This the dynamic nature of the driving task and the fact that leads to signs that are intended to be read when they are
2.2 ROAD USERS 19 The two driver characteristics of utmost importance are visual acuity factors and the reaction process. The two overlap, in that reaction requires the use of vision for most driving cues. Understanding how information is received and processed is a key element in the design of roadways and controls. There are other important characteristics as well. Hearing is an important element in the driving task (i.e., horns, emergency vehicle sirens, brakes squealing, etc.). While noting this is important, however, no traffic element can be designed around audio cues, as hearingimpaired and even deaf drivers are licensed. Physical strength may have been important in the past, but the evolution of power-steering and power-braking systems has eliminated this as a major issue, with the possible exception of professional drivers of trucks, buses, and other heavy vehicles. Of course, one of the most important human factors that influences driving is the personality and psychology oE the driver. This, however, is not easily quantified and is difficult to consider in design. It is dealt with primarily through enforcement and licensing procedures that attempt to remove or restrict drivers who periodically display inappropriate tendencies, as indicated by accident and violation experience. 2.2.1 Visual Characteristics of Drivers When drivers initially apply for, or renew, their licenses, they are aslked to take an eye test, administered either by the state motor vehicle agency or by an optometrist or ophthalmologist who fills out an appropriate form for the motor vehicle agency. The test administered is a standard chart-reading exercise that measures static visual ucui~- that is, the ability to see small stationary details clearly. Visual Factors in Driving While certainly an important characteristic, static visual acuity is hardly the only visual factor involved in the driving task. The Trafic Engineering Handbook [I] provides an excellent summary of visual factors involved in driving, as shown in Table 2.1. Many of the other factors listed in Table 2.1 reflect the dynamic nature of the driving task and the fact that most objects to be viewed by drivers are in relative motion with respect to the driver's eyes. As static visual acuity is the only one of these many visual factors that is examined as a prerequisite to issuing a driver's license, traffic engineers must expect and deal with significant variation in many of the other visual characteristics of drivers. Fields of Vision Figure 2.1 illustrates three distinct fields of vision, each of which is important to the driving task [2]: Acute or clear vision cone-3" to 10" around the line of sight; legend can be read only within this narrow field of vision. Fairly clear vision cone-10" to 12" around the line of sight; color and shape can be identified in this field. Peripheral vision-This field may extend up to 90" to the right and left of the centerline of the pupil, and up to 60" above and 70" below the line of sight. Stationary objects are generally not seen in the peripheral vision field, but the movement of objects through this field is detected. These fields of vision, however, are defined for a stationary person. In particular, the peripheral vision field narrows, as speed increases, to as little as 100" at 20 mi/h and to 40" at 60 mih. The driver's visual landscape is both complex and rapidly changing. Approaching objects appear to expand in size, while other vehicles and stationary objects are in relative motion both to the driver and to each other. The typical driver essentially samples the available visual information and selects appropriate cues to make driving decisions. The fields of vision affect a number of traffic engineering practices and functions. Traffic signs, for example, are placed so that they can be read within the acute vision field without requiring drivers to change their line of sight. Thus, they are generally placed within a 10" range of the driver's expected line of sight, which is assumed to be in line with the highway alignment. This leads to signs that are intended to be read when they are
20 CHAPTER 2 ROAD USER AND VEHICLE CHARACTERISTICS Table 2.1:Visual Factors in the Driving Task Visual Factor Definition Sample Related Driving Task(s) Accommodation Change in the shape of the lens to Changing focus from dashboard bring images into focus displays to roadwav Static Visual Acuity ability to see small details clearly Reading distant traffic signs. Adaptation Change in sensitivity to different Adjust to changes in light upon levels of light entering a tunnel Angular movement Secing obiects moving across Judging the speed of cars crossing the field of view driverspaths Movement in Depth Detecting changes in visual image size. Judging speed of an approaching vehicle. Color Discrimination between different colors.Identifying the color of signals. Contrast Sensitivity Seeing obiects that are similar in Detecting dark-clothed pedestrians at night. brightness to their background. Depth Perception Judgment of the distance of obiects Passing on two-lane roads with oncoming traffic Dynamic Visual Acuity Ability to see obiects that are in Reading traffic signs while moving motion relative to the eve Eve movement Changing the direction of gaze Scanning the road environment for hazards. Glare Sensitivity Ability to resist and recover from the Reduction in visual performance effects of glare due to headlight glare. Peripheral Vision Detection of obiects at the side of Seeing a bicycle approaching from the left. the visual field. Vergence Angle between the eyes'line of sight. Change from looking at the dashboard to the road. nC2Tm2amaemaaRaaa7Baa a significant distance from the driver:in turn.this im plies how large the sign and its lettering must be in order to be comprehende at that distance.Objects or other Peripheral vision(12-1) cated in the fairly clear and visio Fairly clear (11 Acute vision cone (31) the si Peripheral vision is the single most important fac. Figre.:ustration of Fields of Vision tor when drivers estimate their speed.The movement of objects through the perphera ision field is the 00 driver
20 CHAPTER 2 ROAD USER AND VEHICLE CHARACTERISTICS Table 2.1: Visual Factors in the Driving Task Visual Factor Definition Sample Related Driving Task(s) ~ Accommodation Static Visual Acuity Adaptation Angular Movement Change in the shape of the lens to bring images into focus. Ability to see small details clearly. Change in sensitivity to different levels of light. Seeing objects moving across the field of view. Changing focus from dashboard displays to roadway. Reading distant traffic signs. Adjust to changes in light upon entering a tunnel. Judging the speed of cars crossing drivers’ paths. Movement in Depth Color Discrimination between different colors. Identifying the color of signals. Contrast Sensitivity Detecting changes in visual image size. Seeing objects that are similar in brightness to their background. Judging speed of an approaching vehicle. Detecting dark-clothed pedestrians at night. Depth Perception Judgment of the distance of objects. Passing on two-lane roads with oncoming traffic. Dynamic Visual Acuity Eve Movement Ability to see objects that are in motion relative to the eye. Changing the direction of gaze. Reading traffic signs while moving. Scanning the road environment for hazards. Glare Sensitivity Peripheral Vision Vergence Ability to resist and recover from the effects of glare. Detection of objects at the side of the visual field. Angle between the eyes’ line of sight. Reduction in visual performance due to headlight glare. Seeing a bicycle approaching from the left. Change from looking at the dashboard to the road. (Used with permission of Institute of Transportation Engineers, Dewar, R., “Road Users,” Trafic Engineering Handbook, 5th Edition, Chapter 2, Table 2.2, pg. 8, 1999.) Peripheral vision (120°-180”) Figure 2.1: Illustration of Fields of Vision a significant distance from the driver; in turn, this implies how large the sign and its lettering must be in order to be comprehended at that distance. Objects or other vehicles located in the fairly clear and peripheral vision fields may draw the driver’s attention to an important event occurring in that field, such as the approach of a vehicle on an intersection street or driveway or a child running into the street after a ball. Once noticed, the driver may turn hisher head to examine the details of the situation. Peripheral vision is the single most important factor when drivers estimate their speed. The movement of objects through the peripheral vision field is the driver’s single most important indicator of speed. Old studies have demonstrated time and again that drivers