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?
D. Curves
Imagine you are flying through space. Now imagine that you are flying through space and that a road appears behind you as you fly. You can go straight, turn left or right, or veer up or down. You can keep the ship level and turn right, or you can go straight and veer upwards, or you can turn left and veer downwards. There are all kinds of possibilities.
Now look behind your ship and the road you just made. How would you describe it? Engineers have special terms to describe these turns in the roadway. They are called horizontal curves and vertical curves.
This picture shows the horizonal curve information.
This picture shows the vertical curve information, as well as the begining grade and ending grade of the roadway.
Imagine you are back in your ship. If you keep the ship level and turn left or right, the segment of road is in a horizontal curve.
Now, if you go straight, but turn up or down, the segment of road is a vertical curve.
That's all there is to it. Of course, in real life roads often curve vertically and horizontally at the same time, like the spaceship turning left and going down at the same time. Engineers use mathematical expressions to describe the length and sharpness of these curves.
As we know already, engineers try to make the road as straight as possible. However, in order to follow the landscape and observe all the other factors we have learned about, the road needs vertical and horizontal curves. There are rules that engineers follow when laying out curves to make sure the roadway is safe for traffic. These guidelines are based mainly on traffic speed and visibility.
Horizontal curves, and vertical curves to some extent, are controlled by traffic speed. If the curve is too tight, vehicles will have the tendency to skid off the road when they travel through the curve. This is due to centrifugal force.
Centrifugal force is the tendency for an object traveling in a circular path to move outward from the center of the circle. Have you ever been spun on a merry-go-round? The force pushing you out away from the center of the merry-go-round is centrifugal force.
When you go around a horizontal curve, you can imagine yourself traveling around part of a very large circle. The curve must be gradual enough so that the centrifugal force is not large enough to force your vehicle off the road. Since centrifugal force depends on vehicle speed, higher-speed roads need to have more gradual curves. Have you ever noticed a road sign before a tight curve that warns you to slow down? It's all because of centrifugal force. This is not as much of an issue for vertical curves, as the curve would need to be very sharp and the vehicle traveling very fast for the vehicle to actually fly off the roadway (think of a race car driver).
Vertical and horizontal curves are also controlled by visibility, or sight distance. This is the ability of the driver of a vehicle to see an object (like another car) in the roadway. Engineers must design curves so that drivers are able to see objects far enough away so that they have time to react; such as stopping or swerving to miss it. The time the driver has and the distance it takes for them to stop depend on the speed of the vehicle. This is the reason for no-passing zones on two-lane roads.
The previous section can explain part of the reason we have speed limits on our highways. If you travel well above the speed limit, you could be endangering yourself and others on the roadway.
TO: Unit 4 E. Access
