Geometric design, general concepts

1. Geometric Design:
General Concept
CE331 Transportation Engineering

2. Objectives
 Maximize
 Comfort
 Safety
 Economy
 Minimize
 Environmental impacts
… Design a safe and efficient system
that addresses community and
environmental needs

3. Highway Classification
 Location
 Rural
 Urban
 Functional class
 Arterial (Principal
and Minor
 Collector (Major
and Minor)
 Local
 Designation
 Interstate
 Federal
 State
 County
 Local

4. Functional Class - example
Prin. Arterial
Minor Arterial
Collector

5. Some Characteristics
 Major Arterial
 Minor Arterial
 Collectors
 Local roads
Veh-miles
Lane-miles
Mobility
Accessibility
Think of it like this: as you move from local
roads upwards in functional classification
towards major arterials, what happens to
each of these 4 measures? Answer:
vehicle miles and mobility go up. Lane
miles and accessibility go down.

6. Mobility vs. Accessibility
Arterial
Collectors
Locals
Mobility
Land
Access

7. What?

9. What Affects Highway
Design?
 Human, vehicle characteristics
 Expected volume and its composition
 Topography of the area
 Functional classification
 Level-of-service to be provided
 Land use and environmental factors
 Community concerns
 Funding

10. Basic Terms
 Design Hourly Volume (DHV)
 Expected number of vehicles in one hour in both
directions
 Directional Design Hourly Volume (DDHV)
 Expected number of vehicles in one hour in one
direction
 Average Daily Traffic (ADT)
 Number of vehicles in 24 hours in both directions
 Average Annual Daily Traffic (AADT)
 Number of vehicles in 24 hours in both directions

11. Basic Terms (cont’d)
 Design Speed
 Choice speed to design for while
balancing safety, mobility, community
issues, and environmental concerns
 Operating Speed
 Average speed people drive
with comfort
 Speed limit
 Maximum legal speed

12. Basic Terms (cont’d)
 Design Vehicle
 Group of vehicles selected for
establishing design control on highways
with typical physical and operational
characteristics
 General classes:
• Passenger car
• Bus
• Truck
• Recreational vehicle

13. Design Vehicles
Which one?
Largest with reasonable frequency

14. ?

15. Roadway Cross Section
Shoulder
Shoulder Traveled way Traveled way
Roadway
Roadway
Clear Zone
Clear Zone
Right of Way
Median

16. Cross Section Components

17. Cross Section Components

18. Travel Lanes
 Purpose
 Width
 10-12 feet
 Arterials and freeway 12 feet
 Cross slope
Always?

19. Shoulders
 Purpose
 Refuge area
 Recovery area
 Pavement support
 Width
 2-12 feet
 Slope
 2-4%

20. Median
 Purpose
 Divide traffic
 Recovery area
 Storage area
 Refuge area
 Aesthetics
 Width
 Varied
 Types
 Raised
 Flush
 Depressed

21. Right-Of-Way
 Total land required for the construction of the
highway
 Needs to be wide enough to accommodate all
the elements of highway cross section and
planned widening of the highway, and all
public utility facilities to be installed along the
highway
 Example:
 2-lane urban collector streets: 40-60ft
 2-lane urban arterials: 84ft

22. Highway Alignment
 Not all roads are straight and level
 Vertical components
 Grades
 Curves
 Horizontal components
 Tangents
 Curves