1. DESIGN CONTROLS
Spring 2015

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

3. 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.)

4. 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

5. Design Vehicle Dimensions
Symbol
Height (ft)
Width (ft)
Length (ft)
Passenger Car
PC or PU
4.25 7 19
Single-Unit Truck SU 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

6. 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

7. AAHSTO (2000)

8. AAHSTO (2000)
WB-40

9. 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)

10. 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)

11. 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

12. 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.

13. 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

14. 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

15. 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

16. 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)

17. 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

18. 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

20. 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

21. 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)

22. 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

23. 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

24. 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.)

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

26. 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.)

27. 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

28. 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

29. 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