Iowa DOT's policy
is to be
sensitive to the environment
and foster and enhance
environmental stewardship.
IOWA DEPARTMENT OF TRANSPORTATION
Presents
ROCKS AND ROADS
Unit 4: How are highways designed?
E. Access
Highways are designed differently based on traffic volumes and speed. Smaller capacity, lower-speed highways often have open or free access. This means that intersecting roads cross at grade and are controlled by stop signs. Larger capacity, higher-speed highways are often called limited or controlled access highways, and may also be called freeways. Controlled access highways usually have at least two lanes going in each direction, and traffic should not normally come to a stop or even slow down very much, even when entering or exiting the highway. This is accomplished by an interchange. An interchange design includes ramps that allow traffic to gain speed before entering the highway and also to slow down before coming to a stop. There are many types of interchanges, but two of the most common are called diamond interchanges and cloverleaf interchanges.
Diamond interchanges are used when a lower capacity road crosses a higher capacity road. A bridge is used to separate the two roads at the intersection, often called an overpass. Traffic can exit or enter the higher capacity road without stopping, but it must stop before going onto the lower-capacity road. Diamond interchanges are named this way because of the way they look from the air.
Cloverleaf interchanges are used to move traffic from one high capacity road to another high capacity road. They allow traffic to enter and exit each roadway without stopping. Like a diamond, cloverleaf interchanges also use an overpass to separate the two roads. They are so named because they resemble a four-leave clover from the air.
TO:Unit 4 F. Earthwork and Drainage
