The development of the US Interstate Highway System was the largest civil engineering project in the world with a final cost of more than €100 billion. Although a number of limited access, dual carriageway roads were built in the US during the 1920s and 1930s, the genesis for the creation of the Interstates was the end of the Second World War, when the Supreme Commander of the Allied Forces, Dwight D Eisenhower saw the German Autobahn system in 1945 which influenced his thoughts towards the creation of a similar controlled access, dual carriageway highway system in the US. [caption id="attachment_40075" align="alignright" width="300"] Interstate 10- Atchafalaya Bridge – copyright 2017[/caption] When Eisenhower became president, he facilitated the introduction of the 1956 Federal Highways Act which was the legislative framework that guided the Interstate programme.

Challenging geography

The construction risks associated with the interstate projects reflected the challenging geography of the US in terms of building the Interstates through: the Rocky Mountains, the Colorado Plateau and the Sierra Nevada Mountains in the West; the soft delta areas of the Mississippi River in the South; and the large urban areas on the East and West coasts. The construction risks were addressed through engineering innovation in the areas of tunnelling, bridges, elevated viaducts and piling. The interstates influenced economic development in the US, however, the different political, economic and environmental factors in American society at the time also in turn influenced the planning, design and construction of the Interstates so as to facilitate access and protect existing communities.

Interstate 95 - the key transport corridor for the East Coast

The original cities of the US on the East coast created the demand for an efficient transport link along the coast given the more widespread use of cars and heavy vehicles in the post war boom period. The East Coast corridor contained 70 million persons in the cities from Boston to Washington DC to Miami during the 1960s and accounted for one third of all economic activity in the US economy at the time. [caption id="attachment_40078" align="alignright" width="300"] Interstate I-95 opening 1963. L to R; Delaware Governor Elbert Carvel, President John Kennedy, Maryland Governor Millard Tawes dedicate the new highway. (Division of Motor Vehicles). Copyright 2017[/caption] Between New York and Washington DC there were a number of key interstate projects which contributed towards the development of Interstate 95. The Baltimore to Philadelphia section of I-95 reflected the post war thinking in the geometric design of roads in North America and Western Europe in terms of broad horizontal curves with minimum radii of 700m and maximum vertical gradients of four per cent that would be more acceptable to road users. This section of I-95 was opened on November 14, 1963, by President John F Kennedy which reflected the important political and economic factors influencing the development of the Interstates in the 1960s. Further north, I-95 was routed east of Philadelphia in the dock area of the city known as Penn’s Landing, the original landing area for European immigrants during the 17th century. The original elevated section of roadway had to be revised by engineers to an at grade vertical alignment to address community concerns. At the technical level, the development of I-95 on the soft coastal ground at Penn’s landing required an innovative solution to prevent ground disruption which might contaminate the city water supply pipe. The engineering solution included the installation of cylindrical metal tubes into the contaminated ground which were then cleaned out, and the piles were then driven into the empty cylindrical tubes. The solution ensured that the pile installation would not disturb the underground services. The late Richard Burke worked on the design of I-95 with consultants in New York during the early 1960s and he subsequently used this experience to lead the design team on the M7 Naas Bypass, Ireland’s first motorway, in the 1970s. [caption id="attachment_40076" align="alignright" width="300"] Motorists travel on the newly opened section of the Northeastern Expressway in 1963. (William Mortimer/Baltimore Sun)[/caption] This illustrates the international knowledge sharing in relation to geometric design in terms of the influence of the Interstate design standards on the Western European motorway network and vice versa.

Interstate 75- Michigan to Florida

In parallel with I-95, Interstate 75 was another key north south route on the other side of the Appalachian Mountains from Michigan south to Florida. I-75 served the seasonal tourist traffic to Florida. The community objections in Miami against the proposed route terminal point meant that I-75 had to be located further north of the original route however the new route crossed a wetland national park called Alligator Alley. To account for this the design of I-75 included for animal underpasses to maintain wildlife access within the national park. Further north on I-75, the twin peninsula geography of Michigan near the Great Lakes required the design and construction of the longest suspension bridge in the US to link the peninsulas. [caption id="attachment_40096" align="alignright" width="300"] Interstate 75 Mackinac Bridge – copyright 2017[/caption] The Mackinac Bridge was designed to account for low temperatures and high speed winds in the area; the bridge was designed to sway 8m in high wind conditions. The foundation for the Mackinac Bridge was constructed to penetrate 210 feet into the bedrock under the lakebed.

Interstate 10 on the Gulf Coast

The central region of the US - the Midwest - is served by key east-west interstate routes such as Interstate 10 along the Gulf coast in the south and Interstates 70, 80 and 90 further north. Interstate 10 links the southern states of Florida, Mississippi, Alabama and Louisiana to the Midwest states of Texas and New Mexico. The design of I-10 in the soft ground conditions of the Mississippi River delta required innovative engineering solutions. I-10 was constructed as an elevated viaduct carried on piles driven in the Atchafalaya Swamp, near the Mississippi River delta. The Atchafalaya Bridge (main picture) was 26km in length. The precast segments were sailed into place via the network of canals within the Atchafalaya Swamp and lifted by cranes into place on top of the piled columns. The Atchafalaya Bridge was completed in 1973.

Interstate 70 - engineering highlight of the Interstate System

One of the most interesting sections of the Interstate system Interstate 70 in Colorado and its connection to Interstate 15 in Utah. Three of the top five engineering innovations of the interstate system are located along this section of highway reflecting the challenge of building I- 70 through the Rocky Mountains and the Colorado Plateau. [caption id="attachment_40077" align="alignright" width="300"] Interstate 70 Eisenhower Tunnel – copyright 2017[/caption] The Eisenhower Tunnel, pictured right, on I-70 was designed to traverse through the Rocky Mountains which effectively divided Colorado during the wintertime. In terms of access the twin bore tunnel was constructed at 11,000 feet above sea level; the eastbound bore was initially developed as a smaller pilot bore to obtain geological data for construction of the westbound bore. The geological data indicated that the rock at the tunnel was extremely hard. The harsh working conditions required shift working by 1,000 workers to excavate the 500,000 cubic metres of hard rock in order to construct the tunnel. The technical problems associated with collapsing of the rock face in the tunnel required 190,000 cubic metres of concrete facing to hold the tunnel rock face in place. The water inflow from the rock face of the tunnel required a drainage system to accommodate 300 gallons per minute, the engineers designed the drainage to accommodate 500 gallons per minute. The ventilation requirements for the tunnel was based on the empirical data from emission testing of vehicles, which stipulated the provision of 1.6 cubic feet of air per minute to address the higher emission levels of vehicles at the higher elevation of the Eisenhower Tunnel. The westbound tunnel was opened in 1973 and the eastbound one was finally opened in 1979 at a total cost of $490 million.

Interstate 70 - Glenwood Canyon section

Further west along I70 was the Glenwood Canyon section, where the route had to be accommodated within the vertical faced canyon of the Colorado River which narrowed to 150 metres at certain points. The Sierra Club and Colorado Open Spaces environmental groups brought a legal challenge to stop construction of I-70, which led to court restrictions on the construction methods within the canyon. The river had to be diverted twelve times to incorporate the I-70 cross section into the canyon. At particularly constrained locations within the canyon, the opposite carriageways had to be located vertical relative to each other through the use of elevated viaducts based on slip form construction where the viaduct section is constructed from the top of each support column in a segmented manner. The Interstate engineers were briefed on this construction method, a first of its kind in the US, by French engineers which illustrated the knowledge sharing relationship within the North Atlantic region.

Interstate 70 - Green River Valley section

Further east I-70 traversed Green River Valley in the state of Utah. This section was an interesting example of how geology influenced the road cross section. Such was the scale of the earthworks within the valley it was decided to adopt a single carriageway road cross section to reduce rock excavations to 3.5 million cubic metres. [caption id="attachment_40090" align="alignright" width="290"] Interstate 70- Glenwood Canyon– copyright 2017[/caption] In 1990, the I-70 cross section of Green River Valley was finally upgraded to dual carriageway to meet traffic demand.

Interstate 15- Virgin River Valley

At the western terminal point of I-70, it connects to I-15, a key north- south interstate which links the Midwest interstates to the Pacific coast. Like the Glenwood Canyon section the dual carriageway road section was constructed in a manner to meet the environmental protection requirements in the valley. The solution was to avoid visually inappropriate embankments or cuttings, and opt instead for elevated viaducts for fill sections and retaining walls for cutting sections. Rock cuttings were engineered to incorporate a natural visual appearance. The construction methods for I-70 and I-15 reflected the challenging topography and geology of the Rocky Mountains and Colorado Plateau. The routes were complete by the mid 1970s. The demand for these interstate routes was driven by the new industries of the Sun Belt states (California, Arizona, New Mexico and Nevada) in the 1960s including the aerospace, electronics and defence sectors. [caption id="attachment_40088" align="alignright" width="300"] Interstate 15- River Valley section Arizona– copyright 2017[/caption] These industrial sectors required year round access from east to west, which illustrated the influence of economic parameters on the development of the interstate system.

Interstate 5 on the West Coast

The key North South interstate on the west coast of the US is Interstate 5 which was constructed in the 1950s and 1960s. The development of I-5 in California reflected the westward movement of the population within the US over the 20th century as developments in ventilation technology made these warmer regions more attractive to live in from a lifestyle perspective. While I-5 was welcomed in Southern California, it was viewed in less beneficial terms in the older northern states like Washington and Oregon. The challenge of building I-5 near Seattle centred on the presence of mountains and water bodies either side of the city. The only route option for I-5 was through the centre of Seattle which led to the inevitable political protests. While I-5 was eventually built through the centre of Seattle, it had a dramatic impact in terms of separating residential communities from the business districts. The I-5 case in Seattle illustrated the challenge for the interstate engineers in terms of delivering high quality transport infrastructure with the necessary political support while also minimising environmental damage.


The development of the US interstate system coincided with the post war role of the US as a global economic superpower. The lessons learned included the incorporation of environmental protection features by engineers to meet political and environmental demands through the provision of tunnels and viaducts to negotiate significant topographical features, and route locations which avoided residential areas. The interstates developed at a time when the US was adopting a more global role which drove interstate development at the strategic level, however the practical implementation of the interstates has to address those local concerns that are inevitably associated with highway development worldwide. Author: Paul MacDonald, CEng MIEI, is an executive engineer/training officer with Kildare County Council National Roads Office. He has 20 years’ experience in highways, water and environmental engineering. He is a divisional representative on the Council of Engineers Ireland and is involved in the Engineers Ireland STEPS school liaison programme. MacDonald has a keen interest in topics such as urbanisation in the emerging states, international engineering contracts and project finance.