I'll take a crack at it:

Since the difference between Warrington 20 and Warrington 19 is only 0.5 miles + abit, I think we can safely assume that initially the distance is < 20 miles. Otherwise, the sign that says Warrington 19 would be rounded up back to 20. It should also be greater or equal to 19.5, or the sign would not say Warrington 20.

What we learned from this:@ Warrington 20 Min Distance = 19.5 | Max Distance = 19.99999

Half a mile on @ Warrington 19, Min Distance becomes 19 - abit | Max Distance = 19.49999 - abit

Another half a miile on @ Warrington 18, Min Distance becomes 18.5 - abit, Max Distance = 18.99999 - abit

Now, lets take a look from Warrington 18. 0.5 mile up the road is the sign Warrington 19. The road sign can not be less than 18 miles away (otherwise the sign that says Warrington 19 will be rounded down to 18)

What we learned from this: @Warrington 18 Min Distance = 18 | Max Distance = 18.4999

Based on my shaky logic, combining the minimum distance counting from Warrington 20 with the maximum counting from Warrington 18...the Warrington 18 sign should be between 18.4 and 18.5 miles