Let's use the given information to calculate the percentage of drivers who exceed the speed limit:

First, let's assume there are 100 drivers on the highway for easy calculation.

According to the information, 5% of drivers are fined for exceeding the speed limit. So, 5% of 100 drivers are fined, which is 5 drivers.

It's also mentioned that 80% of the drivers who exceed the speed limit are not fined. So, let's say there are "x" drivers who exceed the speed limit. Out of these, 80% are not fined, which means 20% of them are fined.

We know that 5 drivers are fined, so we can set up the equation:

0.20x = 5

Now, solve for "x":
x = 5 / 0.20 = 25

So, there are 25 drivers who exceed the speed limit out of 100 drivers. To find the percentage, we can calculate:

(25 / 100) * 100% = 25%

Therefore, 25% of drivers on this highway exceed the speed limit.