In the US, the system of expressways is called the Eisenhower Interstate and Defense Highway System. This does not inlcude all the freeway mileage in the US but is a large portion of it.

There are as of October 2002, 46,726 total miles in the system.

Using Bud's figures:
$1 million for 63 miles on I-19.
To replace the signage in just the American Interstates would cost:
$1 million * 741 = $742,000,000.00
Which is about 424,000,000 pounds

Britain has 2,086 miles of Motorways.
Using Bud's figures:
$1 million for 63 miles on I-19
To replace the signage on just the British Motorways would cost:
$1 million * 33.11 = $33,111,111.00
Which is about 19,000,000.00 pounds.

There are:
60,094,648 people in the UK
290,342,554 people in the US

$742,000,000/290,342,554 = $2.56 per person or about 1.46 pounds per person.
$33,111,111/60,094,648 = $0.55 per person or about 32 pence per person.

Thus using Bud's figures and comparing only the Interstates to the Motorways it's $2.00 cheaper for the British person than the American person.

MattS, using the above figures you will see that there are approximately 4.6 times more miles of highway per person in the US than in the UK. That would account for the cost difference.

Also one needs to consider that states like Nebraska which have a low population but high land area, will most likely have a larger burden than states like New Jersey, which are small and have a much larger population per square mile.

Not necessarily, because population density and road density are likely to be correlated.