|Low and intermediate type||0.672||0.758||0.897||0.967||1.041||1.015||1.025||1.03||1.026||1.01||1.043||1.062||1.066||Nd||Nd|
|TOTAL paved and unpaved||3.546||3.690||3.730||3.838||3.860||3.864||3.867||3.884||3.901||3.905||3.907||3.912||3.934||3.958||3.949|
KEY: N = data do not exist.
a 1960-95 data include the 50 states and the District of Columbia; 1996-98 data include the 50 states, District of Columbia, and Puerto Rico.
b Paved mileage includes the following categories: low type (an earth, gravel, or stone roadway that has a bituminous surface course less than 1" thick) ;intermediate type (a mixed bituminous or bituminous penetration roadway on a flexible base having a combined surface and base thickness of less than 7");high-type flexible (a mixed bituminous or bituminous penetration roadway on a flexible base having a combined surface and base thickness of 7" or more; high-type composite (a mixed bituminous or bituminous penetration roadway of more than 1" compacted materialon a rigid base with a combined surface and base thickness of 7" or more; high-type rigid (Portland cement concrete roadwaywith or without a bituminous wearing surface of less than 1".
c Unpaved mileage includes the following categories: unimproved roadways using the natural surface and maintained to permit passability; graded and drained roadways of natural earth aligned and graded to permit reasonably convenient use by motor vehicles, and that have adequate drainage to prevent serious impairment of the road by normal surface water--surface may be stabilized; and soil, gravel, or stone, a graded and drained road with a surface of mixed soil,gravel, crushed stone, slag, shell, etc.--surface may be stabilized. The percentage of unpaved roads that are nonsurfaced, dropped from approximately 42% in the 1960s to about 37% in the first half of the 1970s, to about 32% in 1980 and has held at about 22% since 1985.
d Source no longer sorts data into these particular categories.
NOTES: A public road is any road under the jurisdiction of and maintained by a public authority (federal, state, county, town, or township, local government or instrumentality thereof) and open to public travel. No consistent data on private road mileage are available (although prior to 1980, some nonpublic roadway mileage are included). Some years contain U.S. Department of Transportation, Federal Highway Administration (FHWA) estimates for some states. Numbers may not add due to rounding. Beginning with the 1997 issue of Highway Statistics, FHWA has instituted a new method for creating mileage based tables derived from the Highway Performance Monitoring System (HPMS) data. Previously, adjustments to tables developed from sampled data were made using areawide mileage data provided by the States; these adjustments are now made using the universe totals from the HPMS data set. In addition, rounding and expansion related differences were spread across table cells so that all table-to-table mileage totals on related tables matched precisely. While this cosmetic step makes all functional system table totals match, an unintended result is that the tables are not reproducible from the data set by any other users. As a result, FHWA made a decision to discontinue the spreading process, and users may note minor differences in table-to-table mileage totals. For record purposes, FHWA considers the mileage totals from table HM-20, Public Road Length, Miles by Functional System in Highway Statistics to be the controlling totals should a single value be required. Thus, total mileage in this table does not match that in tables 1-1 and 1-5.
SOURCES: 1960-95: U.S. Department of Transportation, Federal Highway Administration, Highway Statistics Summary to 1995, FHWA-PL-97-009 (Washington, DC: July 1997), table HM-212.
1996-98: Ibid., Highway Statistics (Washington, DC: Annual issues), table HM-12.