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A PURPOSE OF THE HSM. 2. The Highway Safety Manual (HSM) is a resource that provides safety knowledge. 3 and tools in a useful form to facilitate . download the sample spreadsheets and color PDF file of this guide. Transportation Officials (AASTHO) Highway Safety Manual (HSM) and how it can help you. AASHTO's Highway Safety Manual: Quantification of Highway Safety. Priscilla Tobias, PE. Illinois Department of Transportation. State Safety Engineer.

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Federal Department of Transportation/FHWA website to present roadway safety data. For example, the HSM provides a method to quantify changes in crash frequency as a function of cross-sectional features. With this method, the expected. Errata to Highway Safety Manual, 1st Edition. HSME2. 1. February Page . Existing Text. Corrected Text. Volume 1. The term e(–) is used in.

Custom Core Goal Safeguard human health by incorporating science-based quantitative safety analysis processes within project development that will reduce serious injuries and fatalities within the project footprint. Sustainability Linkage Reducing fatal and serious injuries contributes to the social and economic principles by reducing the impacts associated with personal and public property damage, injury, and loss of life. Substantive safety refers to actual or anticipated safety performance as defined by crash frequency and crash severity. Substantive safety reflects the science of safety: objective knowledge built on science-based discoveries of data-driven assessments of the safety impacts of road design, road user actions or behaviors, and vehicle attributes. RSAs qualitatively report on potential road safety issues and identify opportunities for improvements in safety for all road users based on input from designers, traffic engineers, maintenance experts, law enforcement, and human factors experts. RSAs are particularly beneficial at the planning and design stages of project development. Scoring Requirements 2 points.

This will be facilitated as agencies improve the processes used to collect and maintain data for: Purpose and Intended Audience Advancement in Safety Knowledge Scope and Organization Defining the Project Development Process Organization of the Highway Safety Manual There is no such thing as absolute safety. There 5 is risk in all highway transportation. A universal objective is to reduce the number 6 and severity of crashes within the limits of available resources, science, and 7 technology, while meeting legislatively mandated priorities.

The information in the 8 HSM is provided to assist agencies in their effort to integrate safety into their 9 decision-making processes. Specifically, the HSM is written for practitioners at the 10 state, county, metropolitan planning organization MPO , or local level.

The HSMs 11 intended users have an understanding of the transportation safety field through 12 experience, education, or both. This knowledge base includes: The HSM is intended to be a resource document that is used 22 nationwide to help transportation professionals conduct safety analyses in a 23 technically sound and consistent manner thereby improving decisions made based 24 on safety performance. The 27 HSM is neither intended to be, nor does it establish, a legal standard of care for users 28 or professionals as to the information contained herein.

No standard of conduct or 29 any duty toward the public or any person shall be created or imposed by the 30 publication and use or nonuse of the HSM. If conflicts arise between these publications and the HSM, the 36 previously established publications should be given the weight they would otherwise 37 be entitled, if in accordance with sound engineering judgment. The HSM may 38 provide needed justification for an exception from previously established 39 publications.

The HSM is not a legal standard of care for users. The HSM does not supersede existing publications. While these predictive analyses are quantitatively and 46 statistically valid, they do not exactly predict a certain outcome at a particular 47 location.

Moreover, they can not be applied without the exercise of sound 48 engineering judgment. The HSM provides an opportunity 66 to consider safety quantitatively along with other typical transportation performance 67 measures. What are quantitative predictive analyses?

What is the difference? Descriptive analyses focus on summarizing and quantifying information about crashes that have occurred at a site i. Predictive analyses focus on estimating the expected average number and severity of crashes at sites with similar geometric and operational characteristics.

The expected and predicted number of crashes by severity can be used for comparisons among different design alternatives. Section 1. The first edition does not address issues such as driver education, law 72 enforcement, and vehicle safety, although it is recognized that these are important 73 considerations within the broad topic of improving highway safety. It explains the relationship 81 of the HSM to planning, design, operations, and maintenance activities.

Part A also 82 presents an overview of human factors principles for road safety, and fundamentals 83 of the processes and tools described in the HSM.

Content in Chapter 3 Fundamentals 84 provides background information needed prior to applying the predictive method, 85 accident modification factors, or evaluation methods provided in the HSM. This 86 content is the basis for the material in Parts B, C, and D. The chapters in Part A are: It includes methods useful for 93 identifying improvement sites, diagnosis, countermeasure selection, economic 94 appraisal, project prioritization and effectiveness evaluation.

The chapters in Part B 95 are: Part B Chapters 4 through 9 presents the roadway safety management process including tools for conducting network screening analyses. The estimate can be made for existing conditions, alternative conditions, or proposed new roadways.

Development of a Comprehensive Database System for Safety Analyst

The predictive method is applied to a given time period, traffic volume, and constant geometric design characteristics of the roadway. The Part C predictive method is most applicable when developing and assessing multiple solutions for a specific location. For example, a roadway project that is considering varying cross-section alternatives could use Part C to assess the expected average crash frequency of each alternative. Part C can also be used as a source for safety performance functions SPFs.

Some of the effects are quantified as accident modification factors AMFs. AMFs quantify the change in expected average crash frequency as a result of modifications to a site. AMFs in general can be used to test alternative design options. Part C Chapters 10 through 12 presents the predictive method for estimating expected average crashes on two- lane rural highways, multilane rural highways, and urban and suburban arterials.

Relationship Among Parts of the HSM Exhibit illustrates the relationship among the four parts of the HSM and how the associated chapters within each part relate to one another. This part presents fundamental knowledge useful throughout the manual. Parts B, C, and D can be used in any order following Part A depending on the purpose of the project or analysis. The chapters within each part can also be used in an order most applicable to a specific project rather than working through each chapter in order.

The dashed line connecting Part C with Chapters 4 and 7 denotes that the safety performance functions in Part C can be calibrated and applied in Chapters 4 and 7.

In general, crash reduction may also be achieved by considering: Although education, enforcement, and emergency medical services are not addressed in the HSM, these are also important factors in reducing crashes and crash severity.

This section further provides example applications of the HSM within the generalized project development process illustrating how to integrate the HSM into various types of projects and activities. Crashes may also be reduced through enforcement and education programs. For the purposes of the HSM, the project development process consists of: The left side of the exhibit depicts the overall project development process.

The right side describes how the HSM is used within each stage of the project development process. The text following Exhibit further explains the relationship between the project development process and the HSM. Relating the Project Development Process to the HSM System planning is the first stage of the project development process and it is the stage in which network infrastructure priorities are identified and assessed.

This stage is an opportunity to identify system safety priorities and to integrate safety with other project types e. Chapter 4 Network Screening is used to identify sites most likely to benefit from safety improvements. Chapter 5 Diagnosis can be used to identify crash patterns to be targeted for improvement at each site. Chapter 6 Select Countermeasures can be used to identify the factors contributing to observed crash patterns and to select corresponding countermeasures.

Chapters 7 Economic Appraisal and Chapter 8 Prioritize Projects are used to prioritize expenditures and ensure the largest crash reductions from improvements throughout the system. Each alternative is evaluated across multiple performance measures, which can include weighing project costs versus project benefits.

These projects can include extensive redesign or design of new facilities e. The result of this stage is a preferred design alternative carried forward into preliminary design. Chapter 6 Select Countermeasures is used to identify the factors contributing to observed crash patterns and to evaluate countermeasures.

Chapters 7 Economic Appraisal can be used to conduct an economic appraisal of countermeasures as part of the overall project costs. The chapters within Part D are a resource to compare the safety implications of different design alternatives, and the Chapters in Part C can be used to predict future safety performance of the alternatives The preliminary design, final design, and construction stage of the project development process includes design iterations and reviews at percent complete, percent complete, percent complete, and percent complete design plans.

As modifications to the preferred design are made, the potential crash effects of those changes can be assessed to confirm that the changes are consistent with the ultimate project goal and intent. Chapter 6 Select Countermeasures and Chapters 7 Economic Appraisal can be used during preliminary design to select countermeasures and conduct an economic appraisal of the design options.

Chapters in Parts C and D are a resource to estimate crash frequencies for different design alternatives. These activities can be conducted from a safety perspective using Chapters 5 Diagnosis to identify crash patterns at an existing location, and Chapter 6 Select Countermeasures and Chapters 7 Economic Appraisal to select and appraise countermeasures. Chapter 9 Safety Effectiveness Evaluation provides methods to conduct a safety effectiveness evaluation of countermeasures.

This can contribute to the implementation or modification of safety policy, and design criteria for future transportation system planning. This information could be used to identify projects for safety funding and opportunities to incorporate safety into previously funded projects or studies.

Identify crash patterns, contributing factors, and countermeasures most likely to reduce crashes. Evaluate the economic validity of individual projects and prioritize projects across a system. Parts C and D Assess the safety performance of design alternatives related to change in roadway cross-section, alignment and intersection configuration or operations. The results of these methods can be used to help reach a preferred alternative that balances multiple performance measures.

This information can be used to select a preferred alternative that balances multiple performance measures. Part D, Chapter 13 - Assess the change in crashes from changing roadway cross section. Operations and Maintenance Signal Timing or Phase Modifications Part D, Chapter 14 Assess the effects that signal timing adjustments can have at individual intersections.

Operations and Maintenance Developing an On-Street Parking Management Plan Part D, Chapter 13 Assess the effects that the presence or absence of on-street parking has on the expected number of crashes for a roadway segment. It can also be used to assess the safety effects of different types of on-street parking. Part D, Chapter 13 and 14 Identify the effects that mitigations to roadway segments Ch 13 and intersections Ch 14 may have on safety.

SUMMARY The HSM contains specific analysis procedures that facilitate integrating safety into roadway planning, design, operations and maintenance decisions based on crash frequency.

The following parts and chapters of the HSM present information, processes and procedures that are tools to help improve safety decision-making and knowledge. The HSM consists of the four parts shown below: Achieving Flexibility in Highway Design.

Flexibility in Highway Design. Federal Highway Administration, U. Driving Task Model Driver Characteristics and Limitations Attention and Information Processing Perception-Reaction Time Speed Choice Positive Guidance Impacts of Road Design on the Driver Intersections and Access Points Divided, Controlled-Access Mainline Undivided Roadways Human Factors and the HSM Driving Task Hierarchy Example Scenarios of Driver Overload Area of Accurate Vision in the Eye Perceived Risk of an Accident and Speed With an understanding of how 3 drivers interact with the roadway, there is more potential for roadways to be 4 designed and constructed in a manner that minimizes human error and associated 5 crashes.

The goal of human factors is to 16 reduce the probability and consequences of human error within systems, and 17 associated injuries and fatalities, by designing with respect to human characteristics 18 and limitations. These errors may not result in crashes because drivers 21 compensate for other drivers errors or because the circumstances are forgiving e. Near misses, or conflicts, are vastly 23 more frequent than crashes. One study found a conflict-to-crash ratio of about 2, 24 to 1 at urban intersections.

In-vehicle and roadway distractions, driver inattentiveness, and driver 29 weariness can lead to errors. A driver can also be overloaded by the information 30 processing required to carry out multiple tasks simultaneously, which may lead to 31 error. To reduce their information load, drivers rely on a-priori knowledge, based on 32 learned patterns of response; therefore, they are more likely to make mistakes when 33 their expectations are not met.

In addition to unintentional errors, drivers sometimes 34 deliberately violate traffic control devices and laws. The three major sub-tasks are: Keeping the vehicle at a desired speed and heading within the lane; 39 Guidance: Interacting with other vehicles following, passing, merging, etc. Following a path from origin to destination by reading guide 43 signs and using landmarks. Chapter 3, Section 3.

The relationship between the sub-tasks can be 46 illustrated in a hierarchical form, as shown in Exhibit The hierarchical 47 relationship is based on the complexity and primacy of each subtask to the overall 48 driving task. The navigation task is the most complex of the subtasks, while the 49 control sub-task forms the basis for conducting the other driving tasks.

This can be achieved when high workload in the sub-tasks of control, 56 guidance, and navigation does not happen simultaneously. Topics include driver attention and 60 information processing ability, vision capability, perception-response time, and 61 speed choice. Attention and I nformation Processing 63 Driver attention and ability to process information is limited.

These limitations 64 can create difficulties because driving requires the division of attention between 65 control tasks, guidance tasks, and navigational tasks. While attention can be switched 66 rapidly from one information source to another, drivers only attend well to one 67 source at a time. For example, drivers can only extract a small proportion of the 68 available information from the road scene.

It has been estimated that more than 69 one billion units of information, each equivalent to the answer to a single yes or no 70 question, are directed at the sensory system in one second. When 74 The driving task includes: As with decision making of any sort, error 76 is possible during this process. A driver may neglect a piece of information that turns 77 out to be critical, while another less-important piece of information was retained.

Each may increase the probability of driver 80 error given human information processing limitations.

Criterion Details

Example Scenarios of Driver Overload 82 83 As shown in Exhibit , traffic conditions and operational situations can 84 overload the user in many ways. Roadway design considerations for reducing driver 85 workload are: For drivers with some degree of experience, driving is 94 a highly automated task. That is, driving can be, and often is, performed while the 95 driver is engaged in thinking about other matters.

Most drivers, especially on a 96 familiar route, have experienced the phenomenon of becoming aware that they have 97 not been paying attention during the last few miles of driving. The less demanding 98 the driving task, the more likely it is that the drivers attention will wander, either 99 through internal preoccupation or through engaging in non-driving tasks. Factors such as increased traffic congestion and increased societal pressure to be productive could also contribute to distracted drivers and inattention.

Inattention may result in inadvertent movements out of the lane, or failure to detect a stop sign, a traffic signal, or a vehicle or pedestrian on a conflicting path at an intersection.

When drivers can rely on past experience to assist with control, guidance, or navigation tasks there is less to process because they only need to process new information. Drivers develop Scenario Example High demands from more than one information source Merging into a high-volume, high-speed freeway traffic stream from a high-speed interchange ramp The need to make a complex decision quickly Stop or go on a yellow signal close to the stop line The need to take in large quantities of information at one time An overhead sign with multiple panels, while driving in an unfamiliar place Overload of information or distractions can increase probability of driver error.

Designing facilities consistent with driver expectations simplifies the driving task. Examples of long-term expectancies that an unfamiliar driver will bring to a new section of roadway include: Vision Approximately 90 percent of the information that drivers use is visual. The following aspects of driver vision are described in this section: It is important for guidance and navigation tasks, which require reading signs and identifying potential objects ahead.

Given that actual driving conditions often vary from the ideal conditions listed above and driver vision varies with age, driver acuity is often assumed to be less than 57 feet per inch of letter height for fonts used on highway guide signs. Contrast sensitivity is the ability to detect small differences in luminance brightness of light between an object and the background. The target object could be a curb, debris on the road, or a pedestrian.

Experimental studies show that even alerted subjects can come as close as 30 feet before detecting a pedestrian in dark clothing standing on the left side of the road. On average, drivers see pedestrians at half the distance at which pedestrians think they can be seen.

However, only a small area of the visual field allows accurate vision. This area of accurate vision includes a cone of about two to four degrees from the focal point see Exhibit The lower-resolution visual field outside the area of accurate vision is referred to as peripheral vision.

Although acuity is reduced, targets of interest can be detected in the low-resolution peripheral vision. Once detected, the eyes shift so that the target is seen using the area of the eye with the most accurate vision.

Area of Accurate Vision in the Eye Targets that drivers need to detect in their peripheral vision include vehicles on an intersecting path, pedestrians, signs, and signals. In general, targets best detected by peripheral vision are objects that are closest to the focal point; that differ greatly from their backgrounds in terms of brightness, color, and texture; that are large; and that are moving.

Studies show the majority of targets are noticed when located less than 10 to 15 degrees from the focal point and that even when targets are conspicuous, glances at angles over 30 degrees are rare.

The more demanding the task, the narrower the visual cone of awareness or the useful field of view, and the less likely the driver is to detect peripheral targets. Targets are seen in high resolution within the central degrees of the field of view.

While carrying out the driving task, the driver is aware of information seen peripherally, within the central 20 to 30 degrees. Relative Visibility of Target Object as Viewed with Peripheral Vision Movement in Depth Numerous driving situations require drivers to estimate movement of vehicles based on the rate of change of visual angle created at the eye by the vehicle. These situations include safe following of a vehicle in traffic, selecting a safe gap on a two- way stop-controlled approach, and passing another vehicle with oncoming traffic and no passing lane.

Exhibit illustrates the relative change of the size of an image at different distances from a viewer. The fact that it is a non-linear relationship is likely the source of the difficulty drivers have in making accurate estimates of closing speed. Drivers have difficulty detecting changes in vehicle speed over a long distance due to the relatively small amount of change in the size of the vehicle that occurs per second.

This is particularly important in overtaking situations on two-lane roadways where drivers must be sensitive to the speed of oncoming vehicles.

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When the oncoming vehicle is at a distance at which a driver might pull out to overtake the vehicle in front, the size of that oncoming vehicle is changing gradually and the driver may not be able to distinguish whether the oncoming vehicle is traveling at a speed above or below that of average vehicles. In overtaking situations such as this, drivers have been shown to accept insufficient time gaps when passing in the face of high-speed vehicles, and to reject sufficient time gaps when passing in the face of other low-speed vehicles.

This safety concern is compounded when drivers are not expecting this situation. One example is on a two-lane rural roadway where a left-turning driver must stop in the through lane to wait for an acceptable gap in opposing traffic. An approaching driver may not detect the stopped vehicle.

In this circumstance the use of turn signals or visibility of brake lights may prove to be a crucial cue for determining that the vehicle is stopped and waiting to turn. Drivers have difficulty detecting the rate of closing speed due to the relatively small amount of change in the size of the vehicle that occurs per second when the vehicle is at a distance. On an open road, study drivers fixated approximately 90 percent of the time within a 4-degree region vertically and horizontally from a point directly ahead of the driver.

This indicates that driver visual search is fairly concentrated. On tangent sections, drivers can gather both path and lateral position information by looking ahead. During curve negotiation, visual demand is essentially doubled, as the location of street sign and roadside information is displaced to the left or to the right from information about lane position.

Eye movement studies show that drivers change their search behavior several seconds prior to the start of the curve. These findings suggest that advisory curve signs placed just prior to the beginning of the approach zone may reduce visual search challenges. The visual search varies with respect to the three types of threats: Vehicles coming from behind require the greatest head movement and are searched for the least.

These searches are conducted by only about 30 percent of pedestrians. Searches for vehicles coming from the side and from ahead are more frequent, and are conducted by approximately 50 and 60 percent of pedestrians, respectively.

Interestingly between 8 and 25 percent of pedestrians at signalized downtown intersections without auditory signals do not look for threats. Perception-Reaction Time Perception-reaction time PRT includes time to detect a target, process the information, decide on a response, and initiate a reaction. Although higher values such as 1. At this stage the driver does not know whether the Perception reaction time is influenced by: However, at night an object which is located several degrees from the line of sight, and which is of low contrast compared to the background, may not be seen for many seconds.

The object cannot be seen until the contrast of the object exceeds the threshold contrast sensitivity of the driver viewing it. As discussed in the next section, identification will be delayed when the object being detected is unfamiliar and unexpected. For example, a low-bed, disabled tractor- trailer with inadequate reflectors blocking a highway at night will be unexpected and hard to identify.

The decision does not involve any action, but rather is a mental process that takes what is known about the situation and determines how the driver will respond. Many decisions are made quickly when the response is obvious. For example, when the driver is a substantial distance from the intersection and the traffic light turns red, minimal time is needed to make the decision.

If, on the other hand, the driver is close to the intersection and the traffic light turns yellow, there is a dilemma: The time to make this stop-or-go decision will be longer given that there are two reasonable options and more information to process. If the driver needs more information, they must search for it. On the other hand, if there is too much information the driver must sort through it to find the essential elements, which may result in unnecessary effort and time.

Decision-making also takes more time when drivers have to determine the nature of unclear information, such as bits of reflection on a road at night.

The bits of reflection may result from various sources, such as harmless debris or a stopped vehicle. Response time is primarily a function of physical ability to act upon the decision and can vary with age, lifestyle athletic, active, or sedentary , and alertness. Guidance for a straight-forward detection situation comes from a study of stopping-sight distance perception- reaction times.

The experiment was conducted in daylight while a driver was cresting a hill and looking at the road at the very moment an object partially blocking the road came into view without warning. Although an object can be within the drivers line of sight for hundreds of feet, there may be insufficient light from low beam headlights, and insufficient contrast between the object and the background for a driver to see it.

Perception-reaction time cannot be considered to start until the object has reached the level of visibility necessary for detection, which varies from driver to driver and is influenced by the drivers state of expectation.

A driving simulator study found that drivers who were anticipating having to respond to pedestrian targets on the road edge took an average of 1. It should be noted that subjects in experiments are abnormally alert, and real-world reaction times could be expected to be longer.

It is dependent on driver vision, conspicuity of a traffic control device or objects ahead, the complexity of the response required, and the urgency of that response. Speed Choice A central aspect of traffic safety is driver speed choice. While speed limits influence driver speed choice, these are not the only or the most important influences.

Understanding these cues can help establish self-regulating speeds with minimal or no enforcement. In experiments where drivers are asked to estimate their travel speed with their peripheral vision blocked only the central field of view can be used , the ability to estimate speed is poor. This is because the view changes very slowly in the center of a road scene.

If, on the other hand, the central portion of the road scene is blocked out, Perception reaction time is not fixed. It is influenced by many factors including: Consequently, if peripheral stimuli are close by, then drivers will feel they are going faster than if they encounter a wide- open situation. In one study, drivers were asked to drive at 60 mph with the speedometer covered. In an open-road situation, the average speed was 57 mph. Several studies examined how removing noise cues influenced travel speed.

While drivers ears were covered with ear muffs they were asked to travel at a particular speed. All drivers underestimated how fast they were going and drove 4 to 6 mph faster than when the usual sound cues were present. This is the experience of leaving a freeway after a long period of driving and having difficulty conforming to the speed limit on an arterial road.

One study required subjects to drive for 20 miles on a freeway and then drop their speeds to 40 mph on an arterial road.

The average speed on the arterial was 50 miles per hour. The adaptation effect was shown to last up to five or six minutes after leaving a freeway, and to occur even after very short periods of high speed. Even though drivers may not have all the information for correctly assessing a safe speed, they respond to what they can see. Drivers tend to drive faster on a straight road with several lanes, wide shoulders, and a wide clear zone, than drivers on a narrow, winding road with no shoulders or a cliff on the side.

For example, speeds on rural highway tangents are related to cross-section and other variables, such as the radius of the curve before and after the tangent, available sight distance, and general terrain. Exhibit shows the relationship between risk perception, speed, various geometric elements, and control devices.

These relationships were obtained from a study in which drivers travelled a section of roadway twice. Each time the speed of the vehicle was recorded. The first time test subjects travelled the roadway they drove the vehicle. The second time the test subjects travelled the roadway, there were passengers in the vehicle making continuous estimates of the risk of a crash.

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Perceived Risk of an Accident and Speed Source: One study recorded the speeds of 40 drivers, unfamiliar with the route, on curves with and without speed plaques. Although driver eye movements were recorded and drivers were found to look at the warning sign, the presence of a speed plaque had no effect on drivers selected speed.

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The sections studied had speed limits that ranged from 25 to 55 mph. Speed limit accounted for 53 percent of the variance in speed, but factors such as alignment, cross-section, median presence, and roadside variables were not found to be statistically significantly related to operating speed. This approach is based on a combination of human factors and traffic engineering principles. Conversely, when drivers are not provided with information in a timely fashion, when they are overloaded with information, or when their expectations are not met, slowed responses and errors may occur.

For example, drivers expect that there are no traffic signals on freeways and Positive guidance approach to road design considers driver limitations, expectations, and engineering principles. If design conforms to those expectancies it reduces the risk of a crash. Short-term expectancies can also be impacted by design decisions. An example of a short-term expectation is that subsequent curves on a section of road are gradual, given that all previous curves were gradual.

Determine the placements of signs according to the importance of information, and avoid presenting the driver with information when and where the information is not essential. Where all the information required by the driver cannot be placed on one sign or on a number of signs at one location, spread the signage along the road so that information is given in small chunks to reduce information load.

Where possible, organize pieces of information into larger units. Say the same thing in more than one way. For example, the stop sign in North America has a unique shape and message, both of which convey the message to stop. A second example of redundancy is to give the same information by using two devices e.

It is not intended to be a comprehensive summary, but is intended to provide examples to help identify opportunities where human factors knowledge can be applied to improve design.

Intersections and Access Points As discussed in Section 2. At intersections, each of these elements presents challenges: The path through the intersection is typically unmarked and may involve turning; Guidance: There are numerous potential conflicts with other vehicles, pedestrians, and cyclists on conflicting paths; and Navigation: Changes in direction are usually made at intersections, and road name signing can be difficult to locate and read in time to accomplish any required lane changes.

In about half of the cases, drivers failed to look, and in about half of the cases, drivers looked but did not see. Drivers following one another can make differing decisions in this dilemma zone. As speed increases, the length of the dilemma zone increases. Additionally, as speed increases, the deceleration required is greater and the probability of a rear-end crash may also increase. Distracting situations could include: Turning movements can lead to crashes at intersections or access points due to the following: Drivers turning left during a permissive green light may not realize that an oncoming vehicle is moving at high speed.

About 40 percent of intersection crashes involve a view blockage. Visual blockages also occur where the offset of left-turn bays results in vehicles in the opposing left-turn lane blocking a left-turning drivers view of an oncoming through vehicle. For example, visibility of the intersection pavement or the crossing traffic may be poor, or drivers may have had the right of way for some distance and the upcoming intersection does not look like a major road requiring a stop. In an urban area where signals are closely spaced, drivers may inadvertently attend to the signal beyond the signal they face.

Drivers approaching at high speeds may become caught in the dilemma zone and continue through a red light. The inadequate search can be on the part of the driver, pedestrian, or bicyclist. In right-turning crashes, pedestrians and drivers have been found to be equally guilty of failure to search.

In left-turning crashes, drivers are more frequently found at fault, likely because the left-turn task is more visually demanding than the right-turn task for the driver.

Pedestrians are at risk because of the time required for drivers to respond and because of the energy involved in collisions, even at low speeds. Relatively small changes in speed can have a large impact on the severity of a pedestrian crash. A pedestrian hit at 40 mph has an percent chance of being killed; at 30 mph the risk is reduced to 45 percent; at 20 mph the risk is reduced to 5 percent. Clothing is often dark, providing little contrast to the background.

Interchanges At interchanges drivers can be traveling at high speeds, and at the same time can be faced with high demands in navigational, guidance, and control tasks.

The number of crashes at interchanges as a result of driver error is influenced by the following elements of design: A pedestrian hit at 40 mph has an percent chance of being killed; at 30 mph the risk of being killed is reduced to 45 percent; at 20 mph the risk of being killed is reduced to 5 percent. Alternatively the freeway is congested or if mainline vehicles are tailgating, it may be difficult for drivers to find an appropriate gap into which to merge.

Drivers may fail to detect slowing vehicles ahead, or vehicles changing lanes in the opposing direction, in time to avoid contact. Drivers may try to complete all three tasks simultaneously thereby increasing their willingness to accept smaller gaps while changing lanes or to decelerate at greater than normal rates.

Also, a tight exit ramp radius or an unusually long vehicle queue extending from the ramp terminal can potentially surprise drivers, leading to run-off-road and rear-end crashes.

Divided, Controlled-Access Mainline Compared to intersections and interchanges, the driving task on a divided, controlled-access mainline is relatively undemanding with respect to control, guidance, and navigational tasks.

This assumes that the mainline has paved shoulders, wide clear zones, and is outside the influence area of interchanges. Sleepiness is strongly associated with time of day. It is particularly difficult for drivers to resist falling asleep in the early-morning hours 2 to 6 a. Sleepiness arises from the common practices of reduced sleep and working shifts. Sleepiness also results from alcohol and other drug use. They provide strong auditory and tactile feedback to drivers whose cars drift off the road because of inattention or impairment.

Drivers limitations in perceiving closing speed result in a short time to respond once the driver realizes the rapidity of the closure. Alternatively, drivers may be visually attending to the vehicle directly ahead of them and may not notice lane changes occurring beyond.

If the lead driver is the first to encounter the stopped vehicle, realizes the situation just in time, and moves rapidly out of the lane, the stopped vehicle is uncovered at the last second, leaving the following driver with little time to respond. Such crashes may occur because an animal enters the road immediately in front of the driver leaving little or no time for the driver to detect or avoid it.

Low conspicuity of animals is also a problem. Given the similarity in coloring and reflectance between pedestrians and animals, the same driver limitations can be expected to apply to animals as to pedestrians in dark clothing. Based on data collected for pedestrian targets, the majority of drivers traveling at speeds much greater than 30 mph and with low-beam headlights would not be able to detect an animal in time to stop.

Undivided Roadways Undivided roadways vary greatly in design and therefore in driver workload and perceived risk. Some undivided roadways may have large-radius curves, mostly level grades, paved shoulders, and wide clear zones. On the other hand, undivided roadways may be very challenging in design, with tight curves, steep grades, little or no shoulder, and no clear zone. In this case the driving task is considerably more demanding.

On an undivided highway, these problems lead to run-off-road and head-on crashes. Rumble strips are effective in alerting drivers about to leave the lane, and have been shown to be effective in reducing run-off-road and cross-centerline crashes, respectively.

Contrary to some expectations, only about 4 percent of head-on crashes are related to overtaking. Although overtaking crashes are infrequent, they have a much higher risk of injury and fatality than other crashes. As discussed previously, drivers are very limited in their ability to perceive their closing speed to oncoming traffic. They tend to select gaps based more on distance than on speed, leading to inadequate gaps when the oncoming vehicle is traveling substantially faster than the speed limit.

Passing lanes and four- Errors that can lead to crashes on an undivided roadway includes: Treatments which improve delineation are often applied under the assumption that run-off-road crashes occur because the driver did not have adequate information about the direction of the road path.

However, studies have not supported this assumption. The core elements of the driving task were outlined and related to human ability so as to identify areas where humans may not always successfully complete the tasks. There is potential to reduce driver error and associated crashes by accounting for the following driver characteristics and limitations described in the chapter: Drivers can only process a limited amount of information and often rely on past experience to manage the amount of new information they must process while driving.

Drivers can process information best when it is presented: Approximately 90 percent of the information used by a driver is obtained visually.

The amount of time and distance needed by one driver to respond to a stimulus e. Drivers use perceptual and road message cues to determine a speed they perceive to be safe. Information taken in through peripheral vision may lead drivers to speed up or slow down depending on the distance from the vehicle to the roadside objects.

The positive guidance approach is based on the central principle that road design that corresponds with driver limitations and expectations increases the likelihood of drivers responding to situations and information correctly and quickly. When drivers are not provided or do not accept information in a timely fashion, when they are overloaded with information, or when their expectations are not met, slowed responses and errors may occur.

Parts B, C, and D of the HSM provide specific guidance on the roadway safety management process, estimating safety effects of design alternatives, and predicting safety on different facilities. Applying human factors considerations to these activities can improve decision making and design considerations in analyzing and developing safer roads.

Alexander, G. Publication No. Department of Transportation, Washington, DC. Positive guidance in traffic control. Department of Transportation, Washington, DC, Allen, M. Hazlett, H. Tacker, and B. Actual pedestrian visibility and the pedestrian's estimate of his own visibility. Bared, J. Edara, and T. Safety impact of interchange spacing on urban freeways.

Bjorkman, M. An exploration study of predictive judgments in a traffic situation. Campbell, J. Richard, and J. Cirillo, J. Dietz, and P. Analysis and modelling of relationships between accidents and the geometric and traffic characteristics of Interstate system. Cole, B. A field trial of attention and search conspicuity. Dewar, R. Human Factors in Traffic Safety.

Evans, L. Automobile speed estimation using movie-film simulation. Ergonomics, Vol. Speed estimation from a moving automobile. Fambro, D. Fitzpatrick, and R. National Cooperative Highway Research Report Determination of Stopping Sight Distances. Farber, E. Silver, Knowledge of oncoming car speed as determiner of driver's passing behavior.

Highway Research Record, Vol.

Fitzpatrick, K. Carlson, M. Wooldridge, and M. Design factors that affect driver speed on suburban arterials. Habib, P. Pedestrian Safety: The hazards of left-turning vehicles. ITE Journal, Vol.

Hills, B. Visions, visibility and perception in driving. Perception, Vol. IBI Group. A Canadian review. A report prepared for Transport Canada, Krammes, R. Brackett, M. Shafer, J. Ottesen, I. Anderson, K. Fink, O. Horizontal alignment design consistency for rural two- lane highways.

Kuciemba, S. Safety Effectiveness of Highway Design Features: Volume V - Intersections. Lemer, N. Huey, H. McGee, and A. Older driver perception-reaction time for intersection sight distance and object detection. Lerner, N. Williams, and C. Risk perception in highway driving: Lunenfeld, H. Mace, D. Garvey, and R. Relative visibility of increased legend size vs. Department of Transportation, McCormick, E.

Human Factors in Engineering. Mourant, R.

Pdf highway safety manual

Rockwell, and N. Drivers' eye movements and visual workload. Highway Research Record, No. Older, J. Traffic Conflicts - A development in Accident Research. Human Factors, Vol.

Volume 18, No. Olson, P. Forensic Aspects of Driver Perception and Response: Cleveland P. Fancher, and L. Parameters affecting stopping sight distance. Improved low-beam photometrics. Pasanen, E. Driving Speed and Pedestrian Safety: A Mathematical Model. Polus, A. Fitzpatrick, and D. Predicting operating speeds on tangent sections of two-lane rural highways.

Transportation Research Record, Vol. Ranney, T. Masalonis, and L. Immediate and long-term effects of glare from following vehicles on target detection in driving simulator. In Transportation Research Record, Vol. Rockwell, T. Spare visual capacity in driving - revisited.

Vision in Vehicles II. Gale et al Eds. Elsevier Science Publishers B. Salvatore, S. The estimation of vehicle velocity as a function of visual stimulation. Schmidt, F. Distortion of driver' estimates of automobile speed as a function of speed adaptation. Journal of Applied Psychology, Vol. Shinar, D. McDowell, and T. Eye movements in curve negotiation. Smiley, A. Smahel, and M. Impact of video advertising on driver fixation patterns.

Summala, H. Rasanen, and J. Bicycle accidents and drivers' visual search at left and right turns. Treat, J. Tumbas, S. McDonald, D. Shinar, R. Hume, R. Stansfin, and N. Tri-level study of the causes of traffic accidents. Van Houten, R. Malenfant, J. Van Houten, and A. Retting, Using auditory pedestrian signals to reduce pedestrian and vehicle conflicts. Zwahlen, H.

Pdf manual highway safety

Advisory speed signs and curve signs and their effect on driver eye scanning and driving performance. In Transportation Research Record Chapter Introduction Crashes as the Basis of Safety Analysis Objective and Subjective Safety Crashes Are Rare and Random Events Crash Contributing Factors Data for Crash Estimation Data Needed for Crash Analysis Limitations of Observed Crash Data Accuracy Limitations Due To Randomness and Change Evolution of Crash Estimation Methods Indirect Safety Measures Crash Estimation using Statistical Methods Overview of the Part C Predictive Method Safety Performance Functions Accident Modification Factors Weighting using the Empirical Bayes Method Limitations of Part C Predictive Method Application of the HSM Effectiveness Evaluation Overview of Effectiveness Evaluation Effectiveness Evaluation Study Types Changes in Objective and Subjective Safety An ebook is one of two file formats that are intended to be used with e-reader devices and apps such as site Kindle or Apple iBooks.

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