I've done similar stuff, just for railways.
If it were my highway system, I'd use a number-by-islands thing. Since the system is broken into parts, I'd apply the first digit to tell what island it's on; then I'd use the second number to identify the particular segment. Thataways, I-45 is the 5th road on the 4th Island. Just my two cents!
That would be a realy good system. And you could identify the road's direction by using odd numbers for north-south and even numbers for east-west as we use it in Germany