Elements of Design
Sight distance is the length of roadway ahead that is visible to the driver. Sight Distance
is the most important element of safe and efficient operation of a vehicle.
Stopping sight distance is the
Local Road Interchanges
1. L-1 Tight Diamond Interchange
1. Least R/W for interchange options,
most suitable for physical, geometric or
2. Simple operations
3. Meets Drivers expectations
1. Intersection spacing does not provide
Horizontal Curves and
For balance in highway design, all geometric elements should be designed to provide
safe continuous operation at a speed likely to be observed under normal conditions.
The physical location of a hi
Basic Design Policies
1. Project Development Philosophy
a. The Project Development process seeks to provide a
degree of mobility to users of the transportation system that
is in balance with social, economic, and environmental
b. The goal is to in
1. Classes of Design Vehicles
Passenger Cars, Buses, Trucks, Recreational
Must also consider bicyclists and pedestrians in all
Designers should consider the largest design vehicle
likely to use the facility or the design vehicle that
Systems and Classifications
1. Classifications Types
Design Type- Functional Classification for design
Classification by Route Numbering for Traffic
Administrative Classifications (NHS vs. Non NHS) for
Grades and Vertical
The grade line is a reference line by which the elevation of the pavement and other
features of the highway are established.
The grade line is controlled mainly by topography, type of highway, performance of
Average Daily Traffic (ADT)
ADT or AADT is the average volume of traffic over a 24 hour
period for a given segment of roadway.
Traffic is counted continuously or estimated based upon adjustments
to periodic counts.
ADTs are not use
Geometric Cross Section
1. Lane Widths
The general lane width on State Highways is 12 feet. AASHTO and Local
Road standards allow 10 and 11 ft. lanes.
For roads with curve radius of 300 feet or less, the lane width shall be
widened to allow for truck of