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Methodology

          There is a lot of conflicting information available regarding county boundary lines and what sections of interstate highway are formally designated or open.  This (unfortunately lengthy) page explains the approach this website has taken in developing the information provided in the constructed tables.

County Boundaries

          For the most part, MapQuest was found to be a reliable source for accurate county boundaries (surprisingly enough).  However, in cases where county boundaries have changed since the early 1990s, MapQuest's boundary lines are inaccurate.  Examples of these changes include those that concerned Staunton and Waynesboro, Virginia.  In Staunton's case in particular, the city's own GIS clearinghouse was brought to bear.

          Other sources relied upon for county boundaries include mob-rule.com, which, for the most part, relies upon the U.S. Census Bureau's TIGER datasets. Google Earth and Live Search Maps were utilized in "non-controversial" situations, so to speak (i.e. problems exist with its county boundary overlays; see the DC area, for one example).

          In terms of determining whether or not an interstate crosses a county boundary, this website only considers the interstate to have "entered" a county if the entire roadway, in both directions, crosses the boundary.  Therefore, and for example, the I-44 flyover that crosses into Wagoner County, Oklahoma is not included in the tables. 

          However, due to consultation with Kendall County, Illinois' own county map, it was determined that both directions of I-80 do indeed clip the southeast corner of that particular county.

Interstate Highway Mileages

The mileages offered are estimates.  With more rigorous research (i.e. and easy access to state DOT route logs), it is certain that more accurate numbers could be calculated.  However, for the core purpose of this website, which was to develop the measure of counties per estimated mileage, minor discrepancies between the estimated mileages and actual mileages were considered tolerable.

          The shortest way of describing of how the mileages were estimated is that this website attempts to have total signed mileage of the interstate highways plus the mileage of unsigned yet formally designated interstate highways.  For example, there is a signed portion of I-695 in Maryland that is actually still MD 695 in reality (i.e. the FHWA Route Log and Finder List does not include the MD 695 mileage in I-695's total).  Another example is that I-495 does not officially concur with I-95 around the Capital Beltway.  But, they are signed as interstates and therefore the mileages were estimated and included.

          Where it was necessary to estimate mileages of interstate highway (as defined above) outside of the FHWA Route Log and Finder List, the following sites were at least consulted:

	Various parts of North Carolina DOT's website
	Tennessee DOT's Official Transportation Map
	New York DOT's Official Description of Highway Touring Routes, Bicycling Touring Routes, Scenic Byways, and Commemorative/Memorial Designations in New York State (October 2007)
	m-plex.com (Clinched Interstate Highway Mapping)
	I-73/I-74 in North Carolina
	The individual route sites on Wikipedia
	Interstate-Guide.com/AARoads

Estimates were also "confirmed" (used loosely) through the use of the routing feature on Google Maps.

          A list of differences between this website's estimated mileages and the FHWA Route Log and Finder List is in the table below.  This list does not include the difference when an additional total was included in the table that was not included in FHWA's list (for example, when the mileages were totaled for only the eastern or western section of I-76, I-84, I-86, etc.):

Also, note that I-35E and I-35W are both included in the tables despite the fact that FHWA includes I-35E in I-35's overall mileage (both incarnations).  However, I-35E's estimated mileage is not double-counted in I-35's total on this website (i.e. I-35's total estimated mileage is equal to FHWA's measurement).  The decision to include I-35E was made in case anyone was interested in those segments of highway alone and due to the fact both segments are signed "I-35E" and not "I-35."

Updates

          It is intended to update this website and its tables as future changes are made to county boundaries and interstate highways.  Therefore, the date on the home page is not static.

Suggestions and Corrections

          As stated in the introduction, there is a lot of conflicting information available.  Therefore, thoughtful suggestions and corrections are encouraged to improve the quality of information provided here.  If you submit a correction, please cite the source of your correction in your e-mail.  Let us not contribute to any more confusion regarding county boundaries and the interstate highways that pass through them.

            Please submit your corrections and suggestions by e-mail.  Thank you in advance for your input.

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