Understanding the interstate route, interstate mile marker, and interchange exit numbering systems improves navigation and safety for motorists, as well as provides a better understanding of the messages shown on Dynamic Message Signs and traffic updates on radio, television, social media and mobile apps.
Interstate Route Numbering
Interstate route numbers provide a clue as to 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. Routes ending with a zero, such as I-70, I-80 and I-90 in Indiana are long-distance, generally transcontinental interstates.
One- or two-digit odd-numbered interstates are north-south routes, with numbers generally increasing from west (I-5 along the Pacific coast) to east (I-95 along the Atlantic coast). Routes ending with a 5 are long-distance north-south interstates, such as 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.
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. 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-469 near Fort Wayne is near the south junction with I-69 and Mile Marker 30 is near the 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. On I-465, a 53-mile circumferential route around Indianapolis, Mile Marker 1 is just west of the I-465 junction with I-65 on the south side of Indianapolis, and Mile Marker 53 is just east of this same southern junction with I-65.
Interstate Interchange Exit Numbers
A third, key indicator of location is the exit number of an interchange, which complements the mile markers on an interstate.
The exit number of an interchange is linked to the mile markers, so that the interchange exit number is the same as the mile marker number. Knowing 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 start a trip at I-65 Exit 103 and your destination is I-65 Exit 253 in northwest Indiana, you know it is approximately 150 miles to your destination. Most maps display interchange s.
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, where many routes cross the highway twice on opposite sides of Indianapolis. Multiple crossings 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 regarding a specific location.