The maximum distance marker I have managed to construct is
>! 468km

Using the following placement of stickers (in **bold** as suggested).

>! 5, 2**5**, 4**5**, 48, 6**7**, 8**7**, 1**07**, 12**6**, 14**6**, 16**6**, 18**5**, 2**04**, 22**4**, 24**4**, 26**3**, 28**3**,   
>! 3**02**, 32**2**, 34**2**, 36**1**, 38**1**, **399**, 4**19**, 42**8**, 44**8**, 46**8**

Progression on the upper bound
>! Combining the digits we have on the existing signs with the digits we have from the stickers gives us a total of 71 digits to work with.  
>! Since we cannot proceed 20, 40, 60, 80,... at the beginning (not enough zeroes) it follows that the signs marked less than 100 will take up at least 9 of these digits. This leaves 62 digits for the 3-digit signs which means that we will be able to produce, at most, **20** signs with 3-digit distances. This gives an absolute upper bound of 499km (in theory our first 3-digit sign could be 119 given what I've said so far).