Did You Know: Interstates and Highways

German autobahn - view from a bridgeAs civil engineers and surveyors, there are many aspects of our daily jobs that are second nature to us. But we hear common questions about our field often from the public. In this “Did You Know” blog series, we’ll answer some of these questions.

We all encounter speed limits, interstate entrance and exit ramps, yield signs and more every day while driving on interstates and highways. Because these are common sights, we often hear questions related to these roadway design features. Who determines speed limits and what is the point of concrete barriers on interstates?

We answer these questions and more in this edition of our “Did You Know” series.

Who determines where passing zones go, and who makes these decisions?

Engineers or governmental agencies determine the locations of passing zones based on factors such as the design speed, sight distance (determined by terrain) and costs along a stretch of roadway. As it relates to costs, in mountainous terrain it may be more economical to build additional intermittent lanes to allow for passing than to add continuous extra lanes. Benefits of allowing motorists to utilize passing lanes are maintaining good traffic flow and average travel speed along a stretch of roadway.

When can yield signs be used instead of stop signs?

Yield signs are used when vehicles can safely perform the maneuver by merging rather than completing a full stop. They typically are installed when the driver has only one direction to visually check for oncoming traffic before making the maneuver. The rule of thumb is that a stop sign can be used anywhere a yield sign can be used, but a yield sign cannot be used everywhere a stop sign is used.

How do you determine the lengths of interstate exit and entrance ramps?

Lengths of ramps are determined by speeds and topography. The speeds of both the interstate and secondary roadway must be reviewed as the length recommendations are based on the differential of the speeds in the two roads to allow sufficient distance for acceleration and deceleration. Topography can affect the ramp lengths because it impacts the distance needed for a heavy vehicle to accelerate uphill or decelerate downhill. The surrounding topography can also be a governing factor in the sufficiency of room allowed for the ramp and whether curves and superelevation are required for the ramp to operate efficiently. Traffic congestion can also affect ramp lengths since queues overflowing on the interstate at exits are a safety concern.

What are the concrete barriers on the interstate for?

The primary purpose for a concrete barrier is to provide a physical barrier to protect vehicles from a hazard. The hazard could mean a drop-off on the side of the road, a threat of crossing into oncoming traffic or an obstruction located close to the roadway such as a large pole for utilities or overhead sign. When the barrier is located in the middle of the interstate, it generally indicates the opposing inside lanes are too close horizontally to satisfy the clear zone requirement. This means there is not sufficient horizontal room to allow an errant vehicle to correct itself and re-enter its own lane at the speed of the road without running into the opposing traffic. A secondary purpose of a barrier located in the middle of the interstate is to act as a sight barrier. If you travel in large cities, such as Atlanta, you might see that the median concrete barrier is taller and you cannot see the lanes traveling in the opposite direction. This helps prevent “rubber necking” which can lead to unnecessary traffic delays and secondary crashes.

Why do you see different color reflectors along the striping on roads?

We commonly refer to these reflectors in Alabama as “Raised Pavement Markers.” They enhance the visibility of the striping and reinforce what the stripe is telling the driver. Striping over time tends to fade and not be quite as visible during the dark and when it’s raining. In these conditions reflectors are quite helpful as they reflect the headlights of vehicles. The spacing and color requirements of the reflectors vary from state to state. The spacing of the reflectors tend to be closer together when the driver must make a new maneuver, such as stop at an intersection or shift for a turn lane. The colors of the reflectors typically match the color of the stripe. Red reflectors are also frequently used. If you think about where we see red on the roads, the color is associated with stop, do not enter or a hazard. Therefore, the color red instinctively gains the driver’s attention. Red reflectors are used when the driver needs to be alerted to a change condition in the lane, such as a lane drop or merge.

Why do the stripes on signs lean different ways on the left and right side of the road at hazard locations?

Stripes on signs are designed to lean, or point, toward the direction of travel. In roadway design, we tend to think about the stripe pointing toward the front of the vehicle. Stripes leaning to the right mean stay to the left of the sign and vice versa if they lean to the left.

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