The 90-degree rule means that a golfer is supposed to drive their cart along the cart path (not in the rough parallel to the cart path) until the cart has reached a point where a 90-degree turn would cause the cart to drive across the fairway directly to the ball.
 
 Let's look at the illustration to see how the rule applies.
 
 The tee area is at the bottom of the picture. The putting green is that light green oval at the top of the picture. And let's imagine that the cart path runs up the right-hand side of the fairway.
 
 Your tee shot lands at position 1. Nice shot! Your ball is in the fairway!
 
 So you drive your cart along the black route, leaving the cart path and making a bee-line straight to the ball. That's not the proper thing to do. As more and more people follow your bad example the ground becomes compacted, the turf becomes unhealthy and the lies becomes tighter, harder, and much more difficult.   Additionally, the cart tracks become increasingly unattractive.
 
 Bad you!
 
 But you're still hitting the ball straight. And your second shot lands at position 2. So you drive the cart along the black route directly to position 2, again causing unnecessary damage to the course.
 
 Folks, the center of the fairway is supposed to be a prime hitting zone. One should be rewarded for accuracy off the tee with a nice fluffy lie and your ball sitting up.
 
 But "no". Not with the way you steer the cart.
 
 Now take the considerate cart driver who selects the red route. The damage to the turf is minimal, more attractive course, easier lies.