How are highway numbers selected, and what do the numbers mean?

Rhode Island has three Interstate routes: I-95 from Westerly to Pawtucket, I-295 from Warwick to the Massachusetts border near Attleboro and I-195 from Providence to the Massachusetts border at Seekonk.

The numbering system for the Interstates was set by the American Association of State Highway and Transportation Officials, but there are some exceptions written into federal law. Generally, Interstates that run north-to-south and vice versa are assigned odd-numbered route numbers. I-95, running from Maine to Florida, is a perfect example of this. Interstate routes that run east-to-west and vice versa use even-numbered route numbers. The closest example can be found with the Massachusetts Turnpike, which doubles as I-90 on the first leg of its cross-country journey to Seattle.

Generally all primary Interstate routes have either a one- or two-digit route numbers. When motorists see a three-digit Interstate number, this means they have entered either a loop or spur from the main route. Those that have an even-numbered number preceding the main route number (e.g. the “2” in I-295) means that the highway will eventually return to the route it is leaving from. These normally are bypass routes to avoid major cities, and are wise choices when traveling in unfamiliar areas during peak travel times.

Conversely, those three-digit Interstate routes beginning with an odd number mean that those routes are spurs that leave the main route and do not return. In some cases they may link to another Interstate, U.S. or state highway route. A few states have exceptions.

The U.S. Highway system route numbering is similar to the Interstate in that odd-numbered routes run generally north-to-south and even-numbered routes run east-to-west. Routes 6 and 44 are local examples of east-to-west routes.

Some interesting bits of trivia:

• Long-haul Interstate routes are generally divisible by five.

• There are only three one-digit Interstate routes: I-4 in Florida, I-5 on the West Coast from southern California to Washington, and I-8 from California to Arizona.

• The most heavily traveled part of the Interstate system is I-405 near San Diego, Calif., which carries about 377,000 vehicles a day. The section with the lightest traffic is I-95 near the Canadian border in Maine, with just 1,880 vehicles a day.

• The longest Interstate is I-90, running 3,020 miles from Boston to Seattle.

• The Interstate System connects 45 of the 50 state capitals, as well as Washington, D.C.

• Most states assign exit numbers that are linked to mileage markers on the Interstate. Rhode Island, six other states and the New Jersey Turnpike are exceptions to this rule, and number their exits sequentially.

• Mileage markers increase on even-numbered Interstates as one travels east and decrease heading west. On odd-numbered Interstate routes, mileage markers increases heading north and decrease heading south. Correspondingly, exit numbers decrease and increase in the same fashion.