Two common effects of driving on open highways are velocitation and highway hypnosis. Velocitation is a phenomenon caused by driving for long periods at high speeds. You've probably experienced velocitation coming off of the highway; the change in speed makes you think that the car is going much slower than it actually is.
Velocitation is dangerous because it can lead you to drive much faster than you intend. Avoid speeding on rural roads or after you exit the freeway by checking your speedometer regularly. This way, you will be able to realize how fast you are going.
When driving on rural highways for a long period of time, be aware of a condition known as "highway hypnosis". This condition results in driving in a dulled, drowsy, trance-like state. You should always be aware of your surroundings. To avoid falling asleep behind the wheel, take frequent breaks. Don't drive for a long period of time and stop if you begin to feel tired.