DESIGN CONTROLS Spring 2015

Information Processing Design Controls Vehicles Drivers Dimensions Driving Task Information Processing Performance Pollution Driver Expectancy

Design Vehicles Four general classes of designed vehicles: Passenger cars: regular cars, SUVs, pickup, etc. Buses: inter-city, city transit, school, etc. Trucks: single-unit, tractor-semitrailar, etc. Recreational vehicles: Motor homes, cars with camper trailers Should now include bicycles whenever bicycle use is allowed on the highway (e.g., on- and off-street bike lanes, etc.)

Design Vehicles The designer should consider the largest design vehicle that is expected to use the facility frequently. General guidelines: Use “passenger car” for parking lots. Use “single-unit truck” for urban intersections Use “city transit buses” for intersections where bus routes are used Use “school buses” for rural intersections Use “semitrailer (WB-20)” for intersections located inside industrial parks

Design Vehicle Dimensions Symbol Height (ft) Width (ft) Length (ft) Passenger Car PC or PU 4.25 7 19 Single-Unit Truck SU 11-13.5 8 30 Intercity Bus BUS-40 12 8.5 40 City Bus BUS-45 45 Semitrailer (inter.) WB-40 13.5 8.0 45.5 WB-50 55 Interstate WB-62 68.5 Motor home MH

Vehicle Characteristics Height Overall height – influences vertical clearance Clearance for overpasses & bridges (always use consistent measurements) Driver eye height – influences sight distance Center of gravity height – influences rollover threshold (higher CG leads to higher risk) Width – influences cross-section elements Length – influences vehicle storage areas (turn bays, parking, etc.) Configuration – influences alignment design

AAHSTO (2000)

AAHSTO (2000) WB-40

Vehicle Performance Weight-to-Horsepower Ratio: influences acceleration/deceleration of vehicles Influences speed maintenance on grades Acceleration/deceleration Influences sight distance, speed change lane Speed maintenance Influences length and steepness of grades Passenger cars usually not a concern (see Exhibits 2-24 & 2-25)

Vehicle Performance Rollover Threshold Definition: maximum lateral acceleration that can be achieved without causing rollover Influences horizontal design alignment Influences roadside design materials (minimize tripping mechanisms) Usually problem for vehicles with high CG; however, vehicle with lower CG can also rollover more often (low energy required for rollover to occur)

Vehicular Pollution Two types of pollutants: Atmospheric Noise Influenced by vehicle mix, vehicle speed, climate (thus, may need to change design accordingly) Unwanted sound that intrudes or interferes with activities such as conversation, reading or sleeping Influenced by operational characteristics of vehicles (tires, engines, horns, aerodynamics, etc.) Usually a problem with trucks. In addition, it may only become a problem once adjacent lands become urbanized

Driver Performance When drivers use a highway designed to be compatible with their capabilities and limitations, their performance is aided. When the design is incompatible with their expectation, the chance of driver error will increase, and crashes or inefficient operation may result.

DRIVER PERFORMANCE Physical Characteristics Eye Position Visual Capabilities Influences Sight Distance Influences Sight Distance & Signing Vehicle Visual Acuity (20/40) Driver’s height Visual Angle (Horizontal and Vertical) Color Blindness

Driver Performance Driving Tasks The driving task depends on the information the drivers receive and how it’s being used Once the information is received, drivers need to make decision and proper action Driving encompasses a number of discrete and interrelated activities The driving task is divided into three components: control, guidance and navigation

DRIVER PERFORMANCE Driving Tasks Control Guidance Navigation Driver’s physical interaction with the vehicle Pretrip planning Route Selection Driver’s maintenance of a safe speed and proper path relative to roadway, traffic controls, and other vehicles: Lane placement Car Following Passing Maneuvers

Driver Performance Driver Information Processing Drivers receive information from all senses, but more than 90 percent is received visually It has been argued that drivers process visual information “serially” (one piece of information at the time); others postulate that drivers can process information in parallel (simultaneously)

Driver Information Processing DRIVER PERFORMANCE Driver Information Processing Visual Search Scan the environment and sample information in short glances in search of needed information Detection Detecting the potential needed source of information Recognition Focus attention whether the source of information is needed Action Implement selected control action

Driver Performance Driver Information Processing Mental workload Definition: Time rate of information processing Demand: Time Rate at which the information must be processed in order to operate a vehicle safely; it varies as a function of roadway, roadside development, and environmental conditions Supply: maximum rate at which a driver can process information; part of the process is used for the driving task Relationship between workload and driver performance (see graph) There is an optimum workload demand Driver performance is degraded at demand levels both below and above optimum

Reaction Time Definition: time to detect and recognize an information source upon and initiate a response It is measured from the instant an information source enters the driver’s field of view to the instant the driver initiates a response Influenced by whether the response is expected or unexpected

Values used for Design: 2.5 seconds SSD ~9.0 seconds DSD (varies) Median Driver Values used for Design: 2.5 seconds SSD ~9.0 seconds DSD (varies)

Reaction Time Factors affecting reaction time Individual Physical characteristics (eye, age, etc.) Driving experience Condition: fatigued/rested, drug/alcohol, attentiveness Roadway Environment Complexity, familiarity, visibility

Driver Expectancy Definition: it is the likelihood that a driver will respond to common situations in predictable ways that the driver has found successful in the past Two types: A Priori : long-term expectations drivers bring to the driving task based upon collective past experience, upbringing, culture and learning Ad Hoc : short-term expectations based upon site-specific practices and situations encountered during a particular trip on a particular roadway

Driver Expectancy Key Considerations: Expectancies are associated with all level of driving tasks Drivers experience problems and errors when their expectancies are violated Drivers should not be surprised More predictable design, information display or traffic operation reduces the chance for errors Use advanced warning signs to minimize problems with driver expectancy (e.g., steep grade, dangerous curves, etc.)

Driver Expectancy Examples: Left-hand exits Lane drops Freeway splits Construction joints that do not follow lane markings Narrow brides Dips

Driver Errors High risk areas (more crashes than expected) may be caused by inefficient information-handling errors Two types: Driver’s capabilities: physical, physiological (e.g., alcohol, etc.) Roadway environment: design features that cause a driver to make an error (e.g., poorly timed yellow light, etc.)

Driver Errors Percentage Causes of Mental Overload High-speed demand of information 36.4 Highly complex information sources 9.7 High workload demands 6.3 Large set of information 3.8 Causes of Mental Underload Low workload demands 24.7 External interference 8.2 Internal interference 6.8 Inattention 3.1 Other 1.0

Novice Drivers Important characteristics Inexperienced Willingness to take higher risk Peer pressure Problem with visual search (tend to fix on one object) Difficulty with anticipated workload

Older Drivers Important characteristics Slower information processing Slower reaction time Slower decision making Visual deterioration Hearing deterioration Difficulty to judge time, speed and distance Side effects of prescription drugs