Turn signals, stop signs, broken yellow lines and merging lanes.

Ah, the rules of the road — so often invoked yet, apparently, so poorly observed.

At some point, most of us had at least a passing familiarity with the simple rules that should govern our driving behavior. At some point we passed the written portion of our state’s driving exam to obtain a license.

But, according to a national study done by GMAC, nearly 20 percent of drivers would fail their state’s test if they took it today.

Which probably explains a lot of the bad driving habits that irk the purists among us today:

Pokey drivers driving in the left-hand lane; impatient drivers passing on the right.

Drivers who do not seem to understand the meaning of a merge sign and barge right into oncoming traffic.

Or drivers who fail to use a turn signal when turning, let alone changing lanes.

The list of driving sins is long.

GMAC found that 18.4 percent of the 5,202 drivers it surveyed nationwide — roughly 38 million licensed Americans — could not pass a written driver’s test.

The national average score was 76.2 percent, with 70 percent considered failing.

Among the survey’s other interesting findings:

• Older drivers do better than younger drivers.

• Males over 45 earned the highest average scores.

• Men in general fared better than women: 78.1 percent of men and 74.4 percent for females.

• New Jersey drivers are the least familiar with the rules of the road, with a nearly 40 percent failure rate.

• Drivers in the Midwest had the highest test scores and the lowest failure rate.

• Mainers were in the middle of the pact, scoring an average of 77.6 percent and ranking 26th in the U.S.

As the survey shows, we could probably all stand to take a refresher course from time to time, or brush up on the Maine Motorist Handbook or Study Guide, which can be found at: tinyurl.com/mainedriverexam