《交通工程学》课程教学资源（书籍文献）Traffic Engineering（3rd Edition）.pdf_P36-P40

2.2 ROAD USERS 21 deprived of peripheral vision (using blinders in experimental cases) and deprived of a working speedometer have little i’dea of how fast they are traveling. 2.2.2 Important Visual Deficits There are a number of visual problems that can affect driver performance and behavior. Unless the condition causes a severe visual disability, drivers affected by various visual deficits often continue to drive. Reference 3 contains an excellent overview and discussion of these. Some of the more common problems involve cataracts, glaucoma, peripheral vision deficits, ocular muscle imbalance, depth perception deficits, and color blindness. :Drivers who have eye surgery to correct a problem may experience temporary or permanent impairments. ‘Other diseases, such as diabetes, can have a significant negative impact on vision if not controlled. Some condlitions, like cataracts and glaucoma, if untreated, can lead to blindness. While color blindness is not the worst of these conditions, it generally causes some difficulties for the affected driver, since color is one of the principal means to impart information. Unfortunately, one of the most common forms of color blindness involves the inability to discern the difference between red and green. In the case of traffic signals, this could have a devastating impact on the safety of such drivers. To ameliorate this difficulty to some degree, some blue pigment has been added to green lights and some yellow pigment has been added to red lights, making them easier to discern by color blind drivers. Also, thLe location of colors on signal heads has long been standardized, with red on the top and green on the bottom of vertical signal heads. On horizontal heads, red is on the lefit and green on the right. Arrow indications are either located on a separate signal head or placed below or to the right of ball indications on a mixed signal head. 2.2.3 Perception-Reaction Time The second critical driver characteristic is perceptionreaction time (PRT). During perception and reaction, there are four distinct processes that the driver must perform [4]: Detection. In this phase, an object or condition of concern enters the driver’s field of vision, and the driver becomes consciously aware that something requiring a response is present. Ident@cation. In this phase, the driver acquires sufficient information concerning the object or condition to allow the consideration of an appropriate response. Decision. Once identification of the object or condition is sufficiently completed, the driver must analyze the information and make a decision about how to respond. Response. After a decision has been reached, the response is now physically implemented by the driver. The total amount of time that this process takes is called the perception-reaction time (PRT). In some of the literature, the four phases are referred to as perception, identification, emotion, and volition, leading to the term “PIEV time.” This text will use PRT, but the reader should understand that this is equivalent to PIEV time. Design Values Like all human characteristics, perception-reaction times vary widely amongst drivers, as do a variety of other factors, including the type and complexity of the event perceived and the environmental conditions at the time of the response. Nevertheless, design values for various applications must be selected. The American Association of State Highway and Transportation Officials (AASHTO) mandates the use of 2.5 seconds for most computations involving braking reactions [5], based upon a number of research studies [6-91. This value is believed to be approximately a 90th percentile criterion (i.e., 90% of all drivers will have a PRT as fast or faster than 2.5 s). For signal timing purposes, the Institute of Transportation Engineers [IO] recommends a PRT time of 1.0 second. Because of the simplicity of the response and the preconditioning of drivers to respond to signals, the PRT time is significantly less than that for a braking response on an open highway. While this is a lower value, it still represents an approximately 85th percentile for the particular situation of responding to a traffic signal. AASHTO criteria, however, recognize that in certain more complex situations, drivers may need considerably

22 CHAPTER 2 ROAD USER AND VEHICLE CHARACTERISTICS more time to react than 1.0 or 2.5 seconds. Situations where drivers must detect and react to unexpected events, or a difficult-to-perceive information source in a cluttered highway environment, or a situation in which there is a likelihood of error involving either information reception, decisions, or actions, all would result in increased PRT times. Some of the examples cited by AASHTO of locations where such situations might exist include complex interchanges and intersections where unusual movements are encountered and changes in highway cross-sections such as toll plazas, lane drops, and areas where the roadway environment is cluttered with visual distractions. Where a collision avoidance maneuver is required, AASHTO criteria call for a PRT of 3.0 seconds for stops on rural roads and 9.1 seconds for stops on urban roads. Where collision avoidance requires speed, path, and/or direction changes, AASHTO recommends a PRT of between 10.2-11.2 seconds on rural roads, 12.1-12.9 seconds on suburban roads, and 14.0-14.5 on urban roads. Complete AASHTO criteria are given in Exhibit 3-3, page 116 of Reference 5. Expectancy The concept of expectancy is important to the driving task and has a significant impact on the perceptionreaction process and PRT. Simply put, drivers will react more quickly to situations they expect to encounter as opposed to those that they do not expect to encounter. There are three different types of expectancies: * Continuity. Experiences of the immediate past are generally expected to continue. Drivers do not, for example, expect the vehicle they are following to suddenly slow down. * Event. Things that have not happened previously will not happen. If no vehicles have been observed entering the roadway from a small driveway over a reasonable period of time, then the driver will assume that none will enter now. e Temporal. When events are cyclic, such as a traffic signal, the longer a given state is observed, drivers will assume that it is more likely that a change will occur. The impact of expectancy on PRT is illustrated in Figure 2.2. This study by Olsen, et al. [I17 in 1984 was a controlled observation of student drivers reacting to a similar hazard when they were unaware that it would appear, and again where they were told to look for it. In a third experiment, a red light was added to the dash to initiate the braking reaction. The PRT under the “expected” situation was consistently about 0.5 seconds faster than under the “unexpected” situation. Given the obvious importance of expectancy on PRT, traffic engineers must strive to avoid designing “unexpected” events into roadway systems and traffic controls. If there are all right-hand ramps on a given freeway, for example, left-hand ramps should be avoided if at all possible. If absolutely required, guide signs must be very carefully designed to alert drivers to the existence and location of the left-hand ramp, so that when they reach it, it is no longer “unexpected.” Total Time, seconds Figure 2.2: Comparison of Perception-Reaction Times Between Expected and Unexpected Events (Used with permission of Transportation Research Board, National Research Council, Olson, P., et al., “Parameters Affecting Stopping Sight Distance,” NCHRP Report 270, Washington DC, 1984.)

2.2 ROAD USERS 3 Other Factors Affecting PRT The importance of this factor is illustrated in the following sam e problem A driver rounds a curve at In general.PRTs increase with a number of factors including (1)age,(2)fatigue,(3)complexity of reac- speed of 60 mi/h and sees a truck overturned on the roadway ahead.How far will the driver's vehicle travel /的 cohol and drugs.The latter are addressed d,=1.47*60*2.5=220.5 The vehicle will travel 220.5 f (approximately with the in ent c of rem ethe driver even engages the ton or this Is I ght e over the more general affects of alcohol and drugs.as well tice hy the driv as aging.on driver characteristics are discussed in a later section. celeration begins only when the brake is engaged-afte the perception-reaction process has been completed Reaction Distance 2.2.4 Pedestrian Characteristics roblems inany he dr ecome and pedestrians.A substantial number of traffic accidt an event or and fatalities involve pedestrians.This is not surprising During this time.the vehicle as in any contact between a ped trian and a vehicle,the course at its initial speed.Only after the foot is applied edestri to the brake pedal does the vehicle begin to slow down ans and vehicles cu edestrians cross the stre nresponse intersections and at mid-block locations At signalize by the u 4。 d is intersections,safe accommodation of pedestrian cross in units of mi/and PRTis in units of seconds,it is con ngs is as cr I as vehicle requirem ab venient to convert speeds to ft/s for use: 1mi*(3280n cration of pedestrians in signal timing. At unsignalized crossing locations,gap-acceptance mi 1.46666.=1.47月 behavior of pedestrians is anothe m ortant consider ap a heptance o th ea the behavior of pedestrians them to cross Thus,the reaction distance may be computed as through. d,=1.47St 2-) Walking Speeds where:d =reaction distance,ft Table 2.2 shows 50th percentile walking speeds for d that thes S=initial speed of vehicle,mi/h pedestrians of various ages.It should be sur reaction time.s spec

2.2 ROAD USERS 23 Other Factors Affecting PRT In general. PRTs increase with a number of factors, including (1) age, (2) fatigue, (3) complexity of reaction, and (4) presence of alcohol and/or drugs in the driver’s system. While these trends are well documented, they are generally accounted for in recommended design values, with the exception of the impact of alcohol and drugs. The latter are addressed primarily through enforcement of ever-stricter DWUDUI laws in the various states, with the intent of removing such drivers froim the system, especially where repeated violations make them a significant safety risk. Some of the more general affects of alcohol and drugs, as well as aging, on driver characteristics are discussed in a later section. Reaction Distance The most cxitical impact of perception-reaction time is the distance the vehicle travels while the driver goes through the process. In the example of a simple braking reaction, thie PRT begins when the driver first becomes aware of an event or object in his or her field of vision and ends when his or her foot is applied to the brake. During this time, the vehicle continues along its original course at its initial speed. Only after the foot is applied to the brakle pedal does the vehicle begin to slow down in response to the stimulus. The reaction distance is simply the PRT multiplied by the initial speed of the vehicle. As speed is generally in units of mi/h and PRT is in units of seconds, it is convenient to convert speeds to ft/s for use: Thus, the reaction distance may be computed as: d, = 1.47St (2-1) where: c!, = reaction distance, ft A: t = reaction time, s = initial speed of vehicle, mi/h The importance of this factor is illustrated in the following sample problem: A driver rounds a curve at a speed of 60 mi/h and sees a truck overturned on the roadway ahead. How far will the driver’s vehicle travel before the driver’s foot reaches the brake? Applying the AASHTO standard of 2.5 s for braking reactions: d, = 1.47 * 60 * 2.5 = 220.5 ft The vehicle will travel 220.5 ft (approximately 11-12 car lengths) before the driver even engages the brake. The implication of this is frightening. If the overturned truck is closer to the vehicle than 220.5 ft when noticed by the driver, not only will the driver hit the truck, he or she will do so at full speed-60 mi/h. Deceleration begins only when the brake is engaged-after the perception-reaction process has been completed. 2.2.4 Pedestrian Characteristics One of the most critical safety problems in any highway and street system involves the interactions of vehicles and pedestrians. A substantial number of traffic accidents and fatalities involve pedestrians. This is not surprising, as in any contact between a pedestrian and a vehicle, the pedestrian is at a significant disadvantage. Virtually all of the interactions between pedestrians and vehicles occur as pedestrians cross the street at intersections and at mid-block locations. At signalized intersections, safe accommodation of pedestrian crossings is as critical as vehicle requirements in establishing an appropriate timing pattern. Pedestrian walking speed in crosswalks is the most important factor in the consideration of pedestrians in signal timing. At unsignalized crossing locations, gap-acceptance behavior of pedestrians is another important consideration. “Gap acceptance” refers to the clear time intervals between vehicles encroaching on the crossing path and the behavior of pedestrians in “accepting” them to cross through. Walking Speeds Table 2.2 shows 50th percentile walking speeds for pedestrians of various ages. It should be noted that these speeds were measured as part of a controlled experiment [I21 and not specifically at intersection or mid-block

24 CHAPTER 2 ROAD USER AND VEHICLE CHARACTERISTICS Table 2.2: 50th Percentile Walking Speeds for Pedestrians of Various Ages Age (years) 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 20-29 30-39 4049 60 + 50-59 50th Percentile Walking Speed (ft/s) Males 2.8 3.5 4.1 4.6 4.8 5 .O 5 .O 5.1 5.5 5.2 5.8 5.3 5.1 5.6 5.2 5.2 4.9 5.7 5.4 5.1 4.9 4.1 Females 3.4 3.4 4.1 4.5 5.0 5.0 5.3 5.4 5.4 5.2 5.7 5.6 5.3 5.3 5.4 5.4 NIA 5.4 5.4 5.3 5.0 4.1 (Compiled from Eubanks, J. and Hill, P., Pedestrian Accident Reconstruction and Litigation, 2nd Edition, Lawyers & Judges Publishing Co., Tucson, AZ, 1999.) crosswalks. Nevertheless, the results are interesting. The standard walking speed used in timing signals is 4.0 ft/s, with 3.5 ft/s recommended where older pedestrians are predominant. Most studies indicate that these standards are reasonable and will accommodate 85% of the pedestrian population. One problem with standard walking speeds involves physically impaired pedestrians. A study of pedestrians with various impairments and assistive devices concluded that average walking speeds for virtually all categories were lower than the standard 4.0 ft/s used in signal timing [13]. Table 2.3 includes some of the results of this study. These and similar results of other studies suggest that more consideration needs to be given to the needs of handicapped pedestrians. Gap Acceptance When a pedestrian crosses at an uncontrolled (either by signals, STOP, or YIELD signs) location, either at an intersection or at a mid-block location, the pedestrian must select an appropriate “gap” in the traffic stream through which to cross. The “gap” in traffic is measured as the time lag between two vehicles in any lane encroaching on the pedestrian’s crossing path. As the pedestrian waits to cross, he or she views gaps and decides whether to “accept” or “reject” the gap for a safe crossing. Some studies have used a gap defined as the distance between the pedestrian and the approaching vehicle at the time the pedestrian begins his or her crossing. An early study [I41 using the latter approach resulted in an 85th percentile gap of approximately 125 ft. Table 2.3: Walking Speeds for Physically Impaired Pedestrians Impairment/Assistive Device Average Walking Speed (ft/s) Cane/Crutch Walker Wheelchair Immobilized Knee Below-Knee Amputee Above-Knee Amputee Hip Arthritis Rheumatoid Arthritis (Knee) 2.62 2.07 3.55 3.50 2.46 1.97 2.44-3.66 2.46 (Compiled from Perry, J., Gait Analysis, McGraw-Hill, New York, NY, 1992.)

2.2 ROAlD USERS 25 Gap acceptance behavior, however, is quite complex and varies with a number of other factors, including the speed of approaching vehicles, the width of the street, the frequency distribution of gaps in the traffic stream, waiting time, and others. Nevertheless, this is an important characteristic that must be considered due to its obvious safety implications. Chapter 18, for example, presents warrants for (conditions justifying) the imposition of traffic signals. One of these is devoted entirely to the safety of pedestrian crossings and to the frequency of adequate gaps in the traffic stream to permit safe crossings. Pedestrian Comprehension of Controls One of the problems in designing controls for pedestrians is generally poor understanding of and poor adherence to such devices. One questionnaire survey of 4,700 pedestrians [I51 detailed many problems of misunderstanding. The proper response to a flashing “DON’T WALK” signal, for example, was not understood by 50% of road users, who thought it meant they should return to the curb from which they started. The meaning of this signal is to not start crossing while it is flashing; it is safe to complete a crossing if the pedestrian has already started to do so. Another study [I61 found1 that violation rates for the solid “DON’T WALK’ signal were higher than 50% in most cities, that the use of the flashing “DON’T WALK” for pedestrian clearance was not well understood, and that pedestrians tend not to use pedestrian-actuated signals. Chapter 20 (on signal timing) discusses some of the problems associated with pedesltrian-actuation buttons and their use that compromise both pedestrian comprehension and the efficiency of the signalization. Since this study was completed, the flashing anid solid “DON’T WALK” signals have been replaced by tlhe Portland orange “raised hand” symbol. Thus., the task of providing for a safe environment for pedestrians is not an easy one. The management and control of conflicts between vehicles and pedestrians remains a difficult one. 2.2.5 Impacts of Drugs and Alcohol on Road Users The effect of drugs and alcohol on drivers has received well-deserved national attention for many years, leading to substantial strengthening of DWI/DUI laws and enforcement. These factors remain, however, a significant contributor to traffic fatalities and accidents. Drivers, however, are not the only road users who contribute to the nation’s accident and fatality statistics. Consider that in 1996, 47.3% of fatal pedestrian accidents involved either a driver or a pedestrian with detectable levels of alcohol in their systems. For this group, 12.0% of the drivers and 32.3% of the pedestrians had blood-alcohol levels above 0.10%, the legal definition of ‘‘drunk” in many states. More telling is that 7% of the drivers and 6% of the pedestrians had detectable alcohol levels below this limit. The importance of these isolated statistics is to make the following point: legal limits for DWVDUI do not define the point at whch alcohol and/or drugs influence the road user. Recognizing this is important for individuals to ensure safe driving, and is now causing many states to reduce their legal limits on alcohol to 0.08%, and for some to consider “zero tolerance” criteria (0.01 %) for new drivers for the first year or two they are licensed. Figure 2.3 is a summary of various studies on the effects of drugs and alcohol on various driving factors. Steerin Difficuhes Become Disoriented and Confused Drive Faster Abru t Stopping and garting Physical Uncoordination Tendency to Become Distracted Drive Closer to Centerline Decision Making Visual Acuity Complex Information Processing 1.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 BAC (%) Definite Impairment Possible Probable Impairment Impairment Figure 2.3: Effects of Blood-Alcohol Level on Driving Tasks (Used with permission of Institute of Transportation Engineers, Blaschke, J.; Dennis, M.; and Creasy, E, “Physical and Psychological Effects of Alcohol and Other Drugs on Drivers,” ITE Journal, 59, Washington DC, 1987.)