The united states highway system is a in the united states numbered within a nationwide grid.

Answer – System of roads
 The United States Numbered Highway System is an integrated system of roads in the contiguous United States numbered within a nationwide grid. This system of roads covers a total distance of 259,032 km. It is often called often called U.S. Routes or U.S. Highways

The mistake is the use of a new metal material with a higher specific heat

Explanation:

The specific heat is the heat per unit mass required to raise the temperature of a body by one degree Celsius. Heat transferred between two materials increases with the time they spend in contact with each other. Also, the heat content of a body depends on its mass. This means that a larger body at a certain temperature will have more heat energy than a lesser body at the same temperature. Using a larger piece of iron means more heat is stored in the iron to be transferred to the metal to be shaped. The only problem is in using a new metal with a higher specific heat. This means that more heat energy will now be needed to raise the temperature of that material to the temperature of the heating metal iron. This effect will decrease the heat transferred between the metal iron and the material, increasing the time required to heat the material before shaping.