The interstate numbers can tell you you're approximate location and where you're headed.

The Interstate highway numbering system forms a grid with numerical designations increasing gradually from low to high - both from west to east and south to north.

national map with highways

If you know the Interstate highway number that you are on or near, you can tell the orientation of your travels, what general part of the country you're in, and whether you're traveling in a generally straight direction or if you're headed in circles.

2 Digit Numbers
 
You can tell the general orientation of your travel by the last number
     
interstate 10 highway sign   Even numbers travel east-west (I-4, I-8, I-10, I-12, etc.)  
     
interstate 35 highway sign   Odd numbers travel north-south (I-5, I-15, I-17, I-19, etc.)  
     
interstate 20 highway sign Routes ending in "0" are major east-west routes (I-10, I-20, I-30, I-40, I-70, I-80, I-90)  
     
interstate 45 highway sign Routes ending in "5" are major north-south routes (I-5, I-15, I-25, I-35, I-55, I-65, I-75, I-85, I-95)  
     
You can tell what part of the country you're in by the first number
     
interstate 20 highway sign Lower numbered routes are generally located in the south and west  
     
interstate 59 highway sign Higher numbered routes are generally located in the north and east  
     
3 Digit Numbers
     
Interstate numbers with three digits are loops around cities and business districts
     
interstate 635 highway sign Loop routes and through routes that generally connect to an interstate highway at either end have an even first digit  
     
interstate 310 highwaysign Spur routes and city routes that may or may not connect to an interstate highway at one end have an odd first digit  
     
8 go back..    

Next time you're traveling down the highway, remember that the highway number is more than just a label for the road that you are on. It can also give you an idea about where you are and where you're headed.