DESIGN CONTROLS

Spring 2015

Design Controls

DriversVehicles

Dimensions

Performance

Pollution

Driving Task

Information Processing

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

Design Vehicle 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 12 8.5 45

Semitrailer (inter.) WB-40 13.5 8.0 45.5

Semitrailer (inter.) WB-50 13.5 8.5 55

Interstate WB-62 13.5 8.5 68.5

Motor home MH 12 8 30

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)

WB-40AAHSTO (2000)

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

• Atmospheric– Influenced by vehicle mix, vehicle speed, climate

(thus, may need to change design accordingly)• Noise

– 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

Visual Acuity (20/40)

Visual Angle (Horizontal and Vertical)

Vehicle

Driver’s height

Color Blindness

Driver PerformanceDriving 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 Navigation

Driver’s physical interaction with the vehicle

Pretrip planning

Route Selection

Guidance

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 PerformanceDriver 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 PERFORMANCE

Driver Information Processing

Scan the environmentand sample information in short glancesin search of needed information

Visual Search

Detecting the potential neededsource of information

Focus attention whether the sourceof information is needed

Implement selected control action

Detection

Recognition

Action

Driver PerformanceDriver 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

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 ErrorsPercentage

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