Curves ahead

by Katie Greenwood

You're driving out in the country at night. The road is straight, straight, straight. And then, whoa! You yank the wheel and stomp on the brake. You're lucky you didn't drive right into the ditch.

"Curves are one of the most dangerous features of highway driving, especially on rural county roads," says Bob Sperry, Local Roads Safety Liaison for the Iowa Local Technical Assistance Program.

In Iowa, 39 percent of fatal crashes result from vehicles running off a road, which can happen when drivers misjudge curves, drive too fast through them, or just don’t see them.

Efforts to keep you safe on curves fall into two categories: prevention and mitigation.

Prevention: the best medicine

County or state highway engineers are in charge of keeping rural roads safe. They work to prevent crashes by making curves easier for you to drive on and by installing warning signs and devices. Often engineers use a combination of approaches on a single curve.

Super-elevation

To make curves easier to drive on, engineers sometimes raise the outside edge of the road around a curve. This is called super-elevation. This increases the friction that your tires make with the road, so you can drive safely through a curve without slowing down a lot.

Delineators

Delineators are short, reflective plastic posts mounted in a series along the road. They are most effective at night because light from your headlights bounces off the posts and helps you see how sharp the curve is. They are often used on less sharp curves.

Signs

Signs are the most noticeable way that county engineers make curves safer. There are a couple different types of signs you have probably seen on curves. There are signs that tell you there is a curve ahead. There are also signs that help you see the sharpness of the curve as you drive through it.

Advance warning

You've probably seen these types of advance warning signs:

Speed advisory signs, like the one below, tell you how slow you should drive around a curve.

The advised speed for each curve is based on a study engineers do called a ball bank indicator study.

In this study, engineers drive through the curve several times at different speeds. With the help of a device called a ball bank indicator, they determine which speed is safe for you to drive on that curve.

Horizontal alignment signs tell you the direction of the curve and how many curves are ahead.

From left to right: Single turn sign, reverse and turn sign, and winding road sign

Chevrons

Chevron signs are those black arrows on a bright yellow background. They are often installed along sharp curves where you might need more help seeing the curve.

Chevron sign

Chevrons are effective during the day and at night.

Crash data from the Iowa Department of Transportation (Iowa DOT) show that chevron signs have reduced all crashes on Iowa curves by 35 percent and fatal crashes by 20 percent.

Where there are already chevrons on curves, several engineers have upgraded to bigger, brighter chevron signs to get your attention.

See the sidebar to learn how maintenance crews find out how many chevron signs a curve needs.

Mitigation: easing the pain

Sometimes, despite the best efforts of engineers and drivers, things can still go wrong. Rain, snow, and ice greatly increase the risk of driving off the road on a curve.

That’s why engineers have developed techniques to minimize the amount of damage in case of a crash. This is called crash mitigation.

Guardrails

Engineers sometimes place guardrails along the edge of the roadway curve to prevent you from driving into a ditch. Guardrails often have reflective tape or paint on them. This also helps you see the curve at night.

Guardrails can reduce the severity of run-off-road crashes where there are other dangerous hazards just off the road—like a steep embankment.

Shoulder and roadside improvements

Engineers make improvements to features off the road in case you head into the ditch.

Engineers sometimes increase the width of the shoulder at sharp curves. This gives you more time to react and correct in case you do begin to veer off the road.

They also make the incline from the shoulder into the ditch less steep. This is called the fore-slope.  In case you do accidentally run off the road, a flatter fore-slope makes entering the ditch smoother and safer.

Finally, engineers attempt to move or remove any dangerous off-road fixed objects, like trees or utility poles, that would do a lot of damage if you crashed into them.

Talk back

Have you ever had a close call on a sharp curve? Tell us about it.