How fast can data travel across the US?

The biggest source of latency in intercontinental networking is the speed of light. It takes a minimum of 36ms for a packet to make it from DC to LA and back again.

One way trip along Fiber Optic Cables in Seconds:
<distance in mile> * ( <refractive index of fiber> / <speed of light> )

Multiply times 1000 to convert from seconds to milliseconds. Double it for the round-trip:
<distance in mile> * ( <refractive index of fiber> / <speed of light> ) * 1000 * 2

Here's latency from Washington, DC to Los Angeles, CA:
2308 * (1.46 / 186000) * 1000 * 2 = 36.23311ms

• Speed of light in a Vacuum (in Miles per Second) = 186000
• Refractive Index of Fiber Optic Cable = 1.46
• Distance (From DC to LA in Miles) = 2308