Understanding Interstate Route Numbering, Mile Markers & Exit Numbering

Understanding the Interstate route numbering system, Interstate Mile Markers, and Interstate Interchange Exit Numbers will result in improved navigation and safety for motorists, as well as a better understanding of the messages shown on Dynamic Message Signs and broadcast on Highway Advisory Radio stations.

Interstate Route Numbering

When traveling in unfamiliar territory, Interstate route numbers will provide a clue as to your location and direction of travel.

One or two digit even-numbered Interstates are east-west routes, the numbers generally increasing from south (I-10) to north (I-94).  In Indiana, a state with many east-west routes, I-64 is in Southern Indiana and I-80, I-90, and I-94 are in Northern Indiana.  Furthermore, routes ending with a “0” are long-distance, generally transcontinental Interstates (I-70, I-80, and I-90 in Indiana).

One or two digit odd-numbered Interstates are north-south routes, with numbers generally increasing from west (I-5 along the West Coast) to east (I-95 along the East Coast).  Routes ending with a “5” are long-distance north-south Interstates (I-65 in Indiana).

Three digit Interstates connect or branch off of mainline Interstates.  If the first of the three digits is an even number, the Interstate usually connects to another Interstate at both ends, most often as a beltway or loop around a city (I-265, I-275, I-465, I-469, and I-865 are Indiana examples).  If the first of the three digits is an odd number, the Interstate is usually a spur route that connects with an Interstate at only one end (I-164 is Indiana’s only spur route).

Interstate Mile Markers

The second important, more specific indicator of location is the Mile Marker.

On one or two digit Interstates, the numbering always begins at the south state line (for north-south Interstates) or the west state line (for east-west Interstates).  Thus, Mile Marker 1 on I-65 is just north of the Kentucky State Line and Mile Marker 261 is near I-65’s northern terminus at I-90 in Northwest Indiana.  Likewise, Mile Marker 1 on I-70 is just east of the Illinois State Line and Mile Marker 156 is just west of the Ohio State Line.  In other words, Mile Marker numbers always get larger as you travel north or east on one or two digit Interstates.

If an Interstate originates within a state, the numbering begins from the location where the road begins in the south or west.  For example, Mile Marker 1 on I-164 near Evansville is near the southern terminus of I-164 at US 41, and Mile Marker 20 is near the northern terminus of I-164 near I-64.  I-469 near Fort Wayne follows the same numbering convention, with Mile Marker 1 near its south junction with I-69 and Mile Marker 30 near its north junction with I-69.  Likewise, I-265 in Southern Indiana near Louisville features Mile Marker 1 near its western terminus with I-64 and Mile Marker 6 near its eastern terminus with I-65.  Finally, Mile Marker 1 of I-865 northwest of Indianapolis is near its western terminus with I-65 and Mile Marker 4 is near its eastern terminus with I-465.

Three digit Interstates with an even first number that form a complete circumferential (circle) bypass around a city feature Mile Markers that are numbered in a clockwise direction, beginning just west of an Interstate that bisects the circumferential route near a south polar location.  In other words, Mile Marker 1 on I-465, a 53-mile circumferential route around Indianapolis, is just west of its junction with I-65 on the south side of Indianapolis (on the South Leg of I-465), and Mile Marker 53 is just east of this same South Leg junction with I-65.  Indiana’s other example is I-275, an 84-mile circumferential around Cincinnati that travels through three states, including just over three miles in Indiana.  Mile Marker 1 on I-275 is just west of its junction with I-71/75 south of Cincinnati in Northern Kentucky, and Mile Marker 84 is just east of this same Northern Kentucky junction with I-71/75.  In Indiana near Lawrenceburg, Mile Marker 15 is near the Kentucky State Line and Mile Marker 18 is near the Ohio State Line.

Interstate Interchange Exit Numbers

A third, key indicator of location is the Exit Number of an interchange, complementing the Mile Markers on an Interstate.

The Exit Number of an interchange is linked to the Mile Markers so that the number on the Mile Marker is the same as the interchange Exit Number.  Knowing the interchange Exit Numbers is a great aid in navigation and trip planning.  For example, Exit 103 on I-65 (Southport Road in Indianapolis) is at or very close to Mile Marker 103 on I-65.  If you are northbound and your destination is Exit 253 on I-65 (US 30 in Northwest Indiana), you know it is 150 miles to your destination.  Most maps display interchange Exit Numbers.

Interchange Exit Numbers are also very valuable if a route crosses an Interstate more than once.  This is especially true on circumferential routes such as I-465, with many routes having two junctions with I-465 on opposite sides of Indianapolis.  Multiple crossing of the same route with one Interstate also occurs in rural areas of Indiana.  For example, I-65 features four interchanges with US 231 (Exit 193, Exit 201, Exit 205, and Exit 247).  Knowing the Exit Number eliminates confusion as to which junction with a cross route you are actually near or looking for.