DESIGN CONTROLS
Spring 2016
Design Controls
Vehicles
Drivers
Dimensions
Driving Task
Performance
Information
Processing
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 offstreet 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)
PC or PU
4.25
7
19
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
MH
12
8
30
Passenger Car
Single-Unit Truck
Motor home
Width (ft) Length (ft)
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
• 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
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
Driver’s physical
interaction with the
vehicle
Navigation
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 PERFORMANCE
Driver Information Processing
Visual Search
Detection
Recognition
Action
Scan the environment
and sample information in short glances
in search of needed information
Detecting the potential needed
source of information
Focus attention whether the source
of information is needed
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
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 sitespecific 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
Highly complex information sources
High workload demands
Large set of information
36.4
9.7
6.3
3.8
Causes of Mental Underload
Low workload demands
External interference
24.7
8.2
Internal interference
Inattention
Other
6.8
3.1
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