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Section 5: Interstate Highway Numbering

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Original Numbering System

The original numbering system for interstate highways established that:

  • north-south routes end in 5, beginning with 5 on the west coast and increasing on routes east of IH-5
  • east-west routes end in zero, beginning with 10 in the south and increasing on routes north of IH-10.
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Current Numbering System

After the initial interstate routes were designated, additional corridors became eligible for interstate highway funding. Since the initial routes had used all the available numbers ending with “0” and “5,” an alternate numbering system was developed using even and odd numbers, as follows:

  • Odd numbers designate routes that generally run in a north-south direction.
  • Even numbers designate routes that generally run in an east-west direction.

The number selected would generally fall in the normal sequence between two existing interstate routes. For example, IH-27 is located between IH-25 (New Mexico, Colorado, Wyoming, etc.) and IH-35 (Texas, Oklahoma, Kansas, etc.).

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Loops

Interstate loops use three digits; the first digit is an even number, while the last two digits are associated with the interstate highway that carries the major traffic stream.

Business Loops. Interstate business loops typically retain the number of the interstate highway traversing the city or town and use a green route sign.

Interstate Loops in Texas. Texas interstate loops (both existing and possible) are as follows:

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Loop Number

Location

IH 210

El Paso (possible)

IH 235

Austin (possible)

IH 410

San Antonio

IH 435

Waco (possible)

IH 610

Houston

IH 635

Dallas

IH 810

Beaumont (possible)

IH 820

Fort Worth



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