The factor that most likely prevents states from widening roads to double-size lanes is the option a) eliminate costs.
the construction of the interstate highway network was authorized by the federal aid highway act of 1956, popularly known as the interstate and defense highways act of 1956.
the interstate highway system was created after the end of world war ii for military defense purposes. the project was requested by the main automotive companies of the time and promoted by president eisenhower being inspired by the german motorway networks (autobahnen).
although the construction of the network continues, it was officially declared finished in 1991 (however, 1.5 miles originally planned were not yet built in 2005. the initial estimated cost for the network was 25 billion dollars in 12 years, the final cost it was 114 and it took 35 years to complete it.
subsequently, the states concentrated on making roads considering the local demands of each state, but the federal funds for the construction of large interstate highways have been discontinued.
other options:
technology: there is better technology now than when the highways were built.
speed limit: each state define the speed limits. wider highways lanes are more secure than normal ones.
driver confusion: drivers must be prepared to drive in both kind of highways.