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In 1998, with the Interstate System essentially complete, FHWA's Office of Engineering compiled information about development of the program. In making this information available to the public, we have not updated the material. All information is as of 1998 when the Office of Engineering compiled the report.
The Dwight D. Eisenhower System of Interstate and Defense Highways
Part VII - Miscellaneous Interstate Facts
Numbering Interstate Routes and Exits
Interstate Route Numbering
The Interstate route marker is a red, white, and blue shield, carrying the word "Interstate", the State name, and the route number. Officials of AASHTO developed the procedure for numbering the routes. Major Interstate routes are designated by one- or two-digit numbers. Routes with odd numbers run north and south, while even numbered run east and west. For north-south routes, the lowest numbers begin in the west, while the lowest numbered east-west routes are in the south. By this method, Interstate Route 5 (I-5) runs north-south along the west coast, while I-10 lies east-west along the southern border.
In two cases, a major route has two parallel or diverging branches. In those cases, each branch is given the designation of the main route, followed by a letter indicating a cardinal direction of travel (east, west, etc). In Texas, for example, I-35 splits at Hillsboro, with I-35E going through Dallas, while I-35W goes through Fort Worth. The two branches merge at Denton to reform I-35. A similar situation exists along I-35 in the Minneapolis-St Paul area of Minnesota.
The major route numbers generally traverse urban areas on the path of the major traffic stream. Generally, this major traffic stream will be the shortest and most direct line of travel. Connecting Interstate routes and full or partial circumferential beltways around or within urban areas carry a three-digit number. These routes are designated with the number of the main route and an even-numbered prefix. Supplemental radial and spur routes, connecting with the main route at one end, also carry a three-digit number, using the number of the main route with an odd-number prefix.
To prevent duplication within a State, a progression of prefixes is used for the three-digit numbers. For example, if I-80 runs through three cities in a State, circumferential routes around these cities would be numbered as I-280, I-480, and I-680. The same system would be used for spur routes into the three cities, with routes being numbered I-180, I-380, and I-580, respectively. This system is not carried across State lines. As a result, several cities in different States along I-80 may each have circumferential beltways numbered as I-280 or spur routes numbered as I-180.
Interstate Exit Numbers
The States typically use one of two methods of numbering the Interstate interchange exits.
- The Consecutive numbering system -- Starting at the most westerly or southerly point on each Interstate route, interchanges are numbered consecutively. Thus the first interchange becomes Interchange #1. Each succeeding interchange is numbered consecutively as #2, 3, 4, etc.
- The Milepost numbering system -- All Interstate routes are mileposted beginning at the most westerly or southerly point. The beginning point is milepost '0'. If the first interchange on the route is located between milepost 4.0 and 5.0, it is numbered as Interchange #4. The next interchange, if located at milepost 8.7, would be numbered as Interchange #8, etc. With this system the motorist can easily determine the location and distance to a desired interchange.
Fun Facts
Cost
- As reported to Congress in the 1991 Interstate Cost Estimate, the cost to construct the Interstate System (including preliminary engineering, right-of-way acquisition, and construction was $128.9 billion, of which $114.3 billion was the Federal share.
- The System cost can be broken down into:
| Preliminary Engineering | $ 5.619 billion ( 4.5%) |
| Right of Way | 16.246 billion (13.1%) |
| Construction | 102.391 billion (82.4%) |
| Subtotal | $124.256 billion (100.0%) |
| FHWA Admin, planning, research | 4.644 billion |
| Total | $128.900 billion |
- Most Costly Routes (Eligible for Interstate Construction Funds Based on 1991 Cost Estimate):
| I-95, Miami, FL to Houlton, ME | $8.0 billion |
| I-90, Seattle, WA to Boston, MA | $7.5 billion |
| I-75, Miami, FL to Sault Ste Marie, MI | $5.1 billion |
| I-10, Los Angeles, CA to Jacksonville, FL | $5.0 billion |
Mileage
- Longest Interstate Routes:
| I-90, Seattle, WA to Boston, MA | 3,085.27 miles |
| I-80, San Francisco, CA to Teaneck, NJ | 2,906.77 miles |
| I-40, Barstow, CA to Wilmington, NC | 2,554.29 miles |
| I-10, Los Angeles, CA to Jacksonville, FL | 2,459.96 miles |
| I-70, Cove Fort, UT to Baltimore, MD | 2,175.46 miles |
- Shortest (2-Digit) Interstate Routes:
| I-97, Annapolis to Baltimore, MD | 17.57 miles |
| I-99, Bedford to Bald Eagle, PA | 53.00 miles * |
| I-73, Emery to Greensboro, NC | 56.70 miles * |
| I-86, I-84 to Pocatello, ID | 63.18 miles |
| I-19, Nogales to Tucson, AZ | 63.35 miles |
| (* Additional miles of I-99 in PA and I-73 in NC are expected to be built) | |
- East-West Transcontinental Routes:
| I-10, Los Angeles, CA to Jacksonville, FL | 2,459.96 miles |
| I-80, San Francisco, CA to Teaneck, NJ | 2,906.77 miles |
| I-90, Seattle, WA to Boston, MA | 3,085.27 miles |
- North-South Transcontinental Routes:
| I-5, San Diego to Blaine, WA | 1,382.04 miles |
| I-15, San Diego, CA to Sweetgrass, MT | 1,436.89 miles |
| I-35/35E/35W, Laredo, TX to Duluth, MN | 1,831.43 miles |
| I-55, New Orleans, LA to Chicago, IL | 943.69 miles |
| I-65, Mobile, AL to Gary, IN | 888.08 miles |
| I-75, Miami, FL to Sault Ste Marie, MI | 1,787.49 miles |
| I-95, Miami, FL to Houlton, ME | 1,892.76 miles |
- States with Most Interstate Mileage:
| Texas | 17 routes | 3,232.04 miles |
| California | 25 routes | 2,453.31 miles |
| Illinois | 23 routes | 2,160.13 miles |
| Pennsylvania | 21 routes | 1,754.55 miles |
| Ohio | 21 routes | 1,565.39 miles |
- States with Most Interstate Routes:
| New York | 1,496.79 miles | 28 routes |
| California | 2,453.31 miles | 25 routes |
| Illinois | 2,160.13 miles | 23 routes |
| Pennsylvania | 1,754.55 miles | 21 routes |
| Ohio | 1,565.39 miles | 21 routes |
- Interstate Routes Which Traverse the Most States:
| I-95 - FL,GA,SC,NC,VA,DC,MD,DE,PA,NJ,NY,CT,RI,MA,NH,ME | 16 States |
| I-90 - WA,ID,MT,WY,SD,MN,WI,IL,IN,OH,PA,NY,PA | 13 States |
| I-80 - CA,NV,UT,WY,NE,IA,IL,IN,OH,PA,NJ | 11 States |
| I-70 - UT,CO,KS,MO,IL,IN,OH,WV,PA,MD | 10 States |
| I-10 - CA,AZ,NM,TX,LA,MS,AL,FL | 8 States |
Other Fun Facts
- State Capitals -- All but four State capitals are directly served by the Interstate System. Those not directly served are Juneau, AK; Dover, DE; Jefferson City, MO; and Pierre, SD.
- Oldest Segments -- The oldest Interstate segments actually predate the establishment of the Interstate system. Early examples include a portion of the Grand Central Parkway in Queens, New York, was opened to traffic in July 1936 and later was incorporated into the Interstate System as I-278. The Pennsylvania Turnpike between Irwin (southeast of Pittsburgh) and Carlisle (west of Harrisburg) was officially opened in October 1940 and is now designated as I-76 and I-70. Other freeways and toll roads were incorporated into the System rather than build new competing Interstate routes.
- Rest Areas -- A exact count of rest areas on the Interstate System is not available. However, a count in 1972 reported 1,214 rest areas in existence. The number still operational today is not expected to differ dramatically from the 1972 figure.
- Interchanges -- An exact count of the number of interchanges on the Interstate System is not available. However, a 1978 count found 14,231 interchanges. This number has likely increased somewhat over the intervening years.
Vehicle Miles Travelled on the Interstate System in the U.S.
| Year | Rural | Urban | Total |
|---|---|---|---|
| 1957 1958 1959 | 3,243 6,264 9,775 | 3,563 6,658 10,222 | 6,806 12,922 19,997 |
| 1960 1961 1962 1963 1964 | 10,514 13,091 22,001 27,536 33,595 | 13,365 16,952 22,180 27,674 33,833 | 23,879 30,043 44,181 55,210 67,428 |
| 1965 1966 1967 1968 1969 | 40,310 48,900 54,847 62,300 71,821 | 40,380 50,414 56,317 63,973 73,195 | 80,690 99,314 111,164 126,273 145,016 |
| 1970 1971 1972 1973 1974 | 79,516 89,542 99,024 107,085 104,621 | 81,532 90,117 100,556 108,462 109,304 | 161,048 179,659 199,580 215,547 213,925 |
| 1975 1976 1977 1978 1979 | 111,980 117,885 126,149 136,125 133,597 | 118,232 132,698 141,639 156,793 159,452 | 230,212 250,583 267,788 292,918 293,049 |
| 1980 1981 1982 1983 1984 | 135,084 139,304 142,546 145,250 149,139 | 161,242 166,479 175,879 192,470 204,304 | 296,326 305,783 318,425 337,720 353,443 |
| 1985 1986 1987 1988 1989 | 154,357 159,498 170,493 181,315 191,085 | 216,188 232,017 244,836 258,695 270,735 | 370,545 391,515 415,329 440,010 461,820 |
| 1990 1991 1992 1993 1994 | 200,173 205,011 205,557 208,308 215,568 | 278,901 285,325 303,265 317,399 330,577 | 479,074 490,336 508,822 525,707 546,145 |
| 1995 1996 | 223,382 232,447 | 341,515 351,937 | 564,897 584,384 |
States With the Heaviest Travelled Interstate System Routes
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| State | Route | City or County | Location | Estimated Cost1 ($Millions) | Number of tubes | Length (Feet) | Toll or Free |
|---|---|---|---|---|---|---|---|
| Alabama | I-10 | Mobile | George Wallace Tunnel (Mobile Bay) | $ 56 | 2 | 3,000 | Free |
| Colorado | I-70 | Grand Junction | Beavertail Mountain | 20 | 2 | 600 | Free |
| Colorado | I-70 | Dillon (Straight Creek) | Eisenhower Memorial Tunnel | 227 | 2 | 9,000 | Free |
| Colorado | I-70 | Glenwood Springs | Glenwood Canyon (Hanging Lake) | 143 | 2 | 3,900 | Free |
| Colorado | I-70 | Glenwood Springs | Glenwood Canyon (Reverse Curve - WBL only) | 9 | 1 | 600 | Free |
| Colorado | I-70 | Clear Creek County | Idaho Springs | 2 | 2 | 900 | Free |
| Hawaii | H-3 | Honolulu | Oahu, Trans-Koolau Mountains | 293 | 2 | 5,000 | Free |
| Maryland | I-95 | Baltimore City | Fort McHenry Tunnel (Patapsco River) | 750 | 2 | 5,400 | Toll |
| Massachusetts | I-90 | Boston | Ted Williams Tunnel (Boston Harbor) | 264 | 4 | 7,200 | Toll |
| Nevada | I-80 | Elko County | Carlin Canyon | 8 | 2 | 1,400 | Free |
| North Carolina | I-40 | Haywood County | MP 4, Pigeon River Gorge (both lanes) | 4 | 2 | 1,059 | Free |
| North Carolina | I-40 | Haywood County | MP 8, Pigeon River Gorge (EBL only) | 2 | 1 | 1,203 | Free |
| Virginia | I-64 | Norfolk-Newport News | Hampton Roads | 95 | 2 | 6,987 | Free |
| Virginia | I-77 | Bland County | Big Walker Mountain | 30 | 2 | 4,200 | Free |
| Virginia | I-77 | Bland, VA - Mercer, WV | East River Mountain | 41 | 2 | 5,700 | Free |
| Virginia | I-264 | Norfolk-Portsmouth | Elizabeth River (Downtown Tunnel)2 | 79 | 2 | 3,350 | Free |
| Virginia | I-664 | Suffolk-Newport News | Monitor-Merrimac Tunnel (Hampton Roads) | 200 | 2 | 4,800 | Free |
| Washington | I-90 | Seattle | Mount Baker Ridge Tunnel | 200 | 1 | 1,500 | Free |
| West Virginia | I-70 | Ohio County | Wheeling Tunnel | 7 | 2 | 1,490 | Free |
| Footnotes: 1Large cost differences between tunnels are partially attributed to the 35-year period over which they were built, 1960s to 1990s. 2One tube was initially built by a toll authority; a second tube was added later with Interstate Construction funds. | |||||||
The Capital Beltway
Washington, D.C. Metropolitan Area
Located in Maryland and Virginia, with a short section crossing the southern tip of Washington, D.C. near the Woodrow Wilson Bridge, the Capital Beltway is a 65-mile loop around our Nation's Capital.
| Length: | Maryland | 42.7 miles |
| Virginia | 21.9 miles | |
| Washington, DC | 0.1 miles | |
| Total | 64.7 miles |
Construction started on the Beltway in the late 1950s. The first segment to be opened to traffic was the Woodrow Wilson Bridge and approaches over the Potomac River south of Washington in December 1961. Other segments were open over the next 2½ years, including the American Legion Bridge over the Potomac River at Cabin John which opened in December 1962. The final segments were opened in August 1964, completing the 65-mile loop.
The Beltway was originally constructed partially as a 4-lane and partially as a 6-lane facility. Over the last 25 years it has been reconstructed to an 8-lane facility over nearly all of its length. Some $0.9 billion in funds were made available by the Transportation Equity Act for the 21st Century for the replacement (and widening) of the Wilson Bridge segment, the last remaining 6-lane segment.
Total cost to construct (including reconstruction to 8 lanes):
| Cost: | Virginia | $177.9 million |
| Maryland | 166.0 million | |
| Washington, DC | 0.2 million | |
| Total | $344.1 million |
Opening Dates (Starting at Wilson Bridge, proceeding clockwise)
- Woodrow Wilson Bridge -- December 1961.
- VA - Wilson Bridge approaches to U.S. 1 -- December 1961.
- VA - U.S. 1 to I-395/I-495 -- April 1964.
- VA - I-395/I-95 to Rte 193 -- December 1961.
- VA - Rte 193 to American Legion Bridge approaches -- December 1962.
- MD - American Legion Bridge to Rte 190 -- December 1962.
- MD - Rte 190 to Rte 355 -- November 1963.
- MD - Rte 355 to I-95/I-495 (College Park) -- August 1964.
- MD - I-95/I-495 (College Park) to I-295 -- August 1964.
- MD - I-295 to Wilson Bridge -- December 1961.
Central Artery / Third Harbor Tunnel (CA/THT)
Boston, Massachusetts
Historical Background
The Central Artery (I-93) / Third Harbor Tunnel (I-90) project in Boston has a long history dating back to the early 1970's. In 1974, Massachusetts was the first State to avail itself of the Interstate Withdrawal-Substitution provisions and withdrew unbuilt Interstate segments including the Inner Belt in Boston.
As a result, with the withdrawal of two Interstate segments in Boston, Massachusetts received over $1.5 billion of Federal funds for substitute highway and transit projects in the Boston urbanized area.
An Interstate segment was retained in Boston to provide a two-lane special purpose route (for use of commercial vehicles) connecting the end of I-90 (the Massachusetts Turnpike) to Logan Airport via a tunnel under Boston Harbor. This remaining segment was what later became the Third Harbor Tunnel portion of the CA/THT project.
The Central Artery portion of the CA/THT project was an elevated freeway which had been built in the mid-1950's entirely with State funds and was incorporated into the Interstate System.
Throughout the 1970's and early 1980's, work on extending I-90 and reconstructing I-93 in Boston was slow. Limited improvements were first started at the northern and southern interchange areas on the I-93.
In the 1978 highway act, Congress provided that portions of the northern interchange improvements at the Charles River and the approaches to the Mystic River Bridge (Route 1) in the Charlestown area in Boston would be eligible for Interstate construction funding. Termed the Central Artery North Area (CANA) project, these improvements in Charlestown were completed at a cost of $288 million.
In 1983, to meet the Congressional deadline for build/no-build decisions on unbuilt Interstate segments, Massachusetts submitted a draft EIS for the CA/THT project. A final EIS was approved for the CA/THT project in 1985.
The 1987 highway act provided that portions of the CA/THT project described as the preferred alternative in the 1983 EIS were eligible for Interstate construction funding. The 1987 act excluded from Interstate Construction funding, the portion of the Central Artery from High Street to Causeway Street. This segment was eligible for funding with other Federal-aid funds.
Since the mid 1980's, the State has incorporated additional improvements into the CA/THT project to meet the concerns of involved local communities, to mitigate environmental impacts, to extend project limits, and improve the existing facilities at approaches to the CA/THT project as originally proposed and to incorporate new features (such as HOV facilities) to improve the design of the project.
The cost of the CA/THT project at the time of the 1987 highway act was estimated by the State to be $3.3 billion. In 1998 the State estimated the total project cost at $10.8 billion.
By far, the CA/THT is largest highway project in the country. As expected, the most costly are projects in urban areas, but none involve an undertaking as extensive and complex as the CA/THT which includes:
- the replacement of the I-93 viaduct with a tunnel in the same location directly beneath the existing structure which must be kept open to traffic,
- construction of the I-90 and I-93 freeway segments which include such extensive use of tunnels and cut-and-cover sections with the attendant disposal of millions of cubic yards of excavated material within a major urban area,
- major bridges; extensive relocations of existing utilities; and designs to avoid existing Amtrak and underground commuter rail and passenger stations, and
- reconstruction and expansion at five major interchanges including connections to Logan Airport, the Massachusetts Turnpike and Route 1, all located within five miles of one other.
