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3dis come in many sizes

The average length of a 3-digit interstate is about 21 miles. Strung together, all 220-plus 3di's would extend from Raleigh, N.C. to Honolulu.

3di shapes and lengths are widely diverse. Some spurs are barely out of sight of their parent interstate before they end at a surface street. But others blur the distinction between 3di and 2di. The 63-mile link between I-84 and I-15 in Idaho is called I-86; but the 95-mile link between I-35 and I-70 in Kansas becomes I-135. Go figure. (However, no 3di is longer than its own parent.)

All lengths are from the "Route Log and Finder List - Interstate Highways, USDOT, 2002", unless noted.

See: shortest 3di's; longest 3di's

Shortest 3di's

Most of the following highways are all shorter than 1.5 miles. Some outliers are included because they met the criteria in the 1998 highway log, which has some differing lengths compared to the 1999 log, even for highways that have not physically changed. The 2002 log lengths match those from 1999, except where highway lengths have physically changed.

  1. 0.70 miles - I-878, New York. Not signed as an interstate.
  2. 0.83 miles - I-315, Montana. Not signed as an interstate.
  3. 1.06 miles - I-375, Michigan. The nation's (current) shortest signed interstate.
  4. 1.09 miles - I-180, Wyoming. The only interstate that is not a freeway.
  5. 1.12 miles - I-895, New York.
  6. 1.20 miles - I-194, North Dakota.
  7. 1.21 miles - I-587, New York.
  8. 1.29 miles - I-395, Florida.
  9. 1.34 miles - I-375, Florida.
  10. 1.39 miles - I-115, Montana.
  11. 1.40 miles - I-345, Texas.
  12. 1.44 miles - I-175, Florida.
  13. 1.49 miles - I-189, Vermont.
  14. 1.50 miles - I-705, Washington State.
  15. 1.57 miles - I-579, Pennsylvania. Was 1.30 miles in 1998.
  16. 1.67 miles - I-381, Virginia. Was 1.17 miles in 1998; the official endpoint(s) changed.
  17. 1.98 miles - I-395, Maryland. In 1998, this was the shortest signed interstate at 0.72 miles. But more recent federal highway logs consider I-395 to be nearly 2 miles long.

Longest 3di's

These highways are all longer than 65 miles (and interstates 86 (west), 19, and 97).

For comparison, the longest "orbital freeway" in the world, in the words of the 2001 Guinness Book of World Records, is the M25 motorway (121.5 miles) encircling London: a "3di" in function if not in name. The longest full beltway in the U. S. is Interstate 275 encircling Cincinnati.

  1. 129.61 miles - I-476, Pennsylvania. Stretches from Philly up to Scranton. Actually, 476 was only 21 miles long (the "Blue Route") until 1996, some enterprising Keystoner convinced Washington to get Interstate designation for Route 9 (the PA Turnpike, Northeast extension).
  2. 121.56 miles - I-495, Massachusetts. Takes you all the way around from the New Hampshire state line to Cape Cod's front door.
  3. 98.72 miles - I-287, New York / New Jersey. The closest thing New York City has to a beltway. Once a bridge across Long Island Sound was proposed.
  4. 95.74 miles - I-135, Kansas. Formerly I-35W; another 3-digit route that works more like a 2-digit route.
  5. ~86 miles - I-540, Arkansas. Formerly a Fort Smith spur, this highway was recently extended north in stages along the US 71 corridor to Bentonville. I-540 may fall back off the list if the proposed I-49 extension takes over the extended segment.
  6. 83.71 miles - I-275, Ohio / Indiana / Kentucky. The only 3-digit interstate to pass through 3 states; though I-295, above, may join the club.
  7. 80.81 miles - I-435, Kansas City.
  8. 80.65 miles - I-196, Michigan.
  9. 76.06 miles - I-390, New York.
  10. 75.63 miles - I-580, California.
  11. 73.77 miles - I-295, New Jersey / Del. May move up in the list and extend into Pennsylvania; this is related to the non-existence of I-95 in a section of the Garden State.
  12. 73.05 miles - I-380, Iowa.
  13. 72.15 miles - I-405, California.
  14. 70.52 miles - I-680, California.
  15. 66.60 miles - I-395, Connecticut / Massachusetts.
  16. 66.38 miles - I-495, New York. The Long Island Expressway. Was once proposed to cross Manhattan.
  17. (65 miles) - I-485, North Carolina. Charlotte's Outerbelt won't be complete until about 2013, but the planned length is 65 miles.
A few longer routes have been planned:
  • ~250 miles - I-175, Tennessee - Kentucky
    This road would parallel I-75 to the west, offering a slightly shorter route between Chattanooga and Lexington, at great expense.
  • 200 miles - Outer Beltway, Cincinnati
    There were rumors circa 1997 about this plan, even with a number: Interstate 875. I haven't heard much about this since then.
  • 200 miles - Outer Perimeter, Atlanta
    This highway would serve outer Atlanta. Sections in the north and east might be built first. Currently there is no interstate number proposed, however.
  • 186 miles - I-840, Tennessee
    The most viable of the outer beltway proposals, I-840 would circle Nashville with an average (divide by 2 pi) 30-mile radius. Less than 60 miles of it is currently open to traffic.

More bemused praise

From memepool, Oct. 7, 1999: "We live in an age of miracles. I wanted to know whether I-395 in Baltimore was the shortest interstate highway in the US. Well, this is 1999, and this is the Web, and there's someone out there who cares sincerely."


  1. Route Log and Finder List - Interstate Highways, USDOT, 1998
  2. Route Log and Finder List - Interstate Highways, USDOT, 1999
  3. Route Log and Finder List - Interstate Highways, USDOT, 2002