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Edit:

The maximum distance marker I have managed to construct is

444km

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

8, 26, 46, 66, 85, 94, 114, 129, 148, 163, 183, 193, 212, 228, 248, 267, 287, 307, 326, 345, 365, 385, 404, 424, 444

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).

Original

I had originally thought I had a solution with distance

468 km

Using the following signs

5, 25, 45, 48, 67, 87, 107, 126, 146, 166, 185, 204, 224, 244, 263, 283,
302, 322, 342, 361, 381, 399, 419, 428, 448, 468

But as Weather Vane correctly pointed out in the comments, I had constructed a new sign using only stickers (399) which is not permitted.

Edit:

The maximum distance marker I have managed to construct is

444 km

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

8, 26, 46, 66, 85, 94, 114, 129, 148, 163, 183, 193, 212, 228, 248, 267, 287, 307, 326, 345, 365, 385, 404, 424, 444

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).

Original

I had originally thought I had a solution with distance

468 km

Using the following signs

5, 25, 45, 48, 67, 87, 107, 126, 146, 166, 185, 204, 224, 244, 263, 283,
302, 322, 342, 361, 381, 399, 419, 428, 448, 468

But as Weather Vane correctly pointed out in the comments, I had constructed a new sign using only stickers (399) which is not permitted.

Edit:

The maximum distance marker I have managed to construct is

443km

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

8, 26, 46, 66, 85, 94, 114, 129, 148, 164, 184, 198, 212, 228, 247, 267, 287, 306, 325, 345, 365, 384, 403, 423, 443

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).

Original

I had originally thought I had a solution with distance

468 km

Using the following signs

5, 25, 45, 48, 67, 87, 107, 126, 146, 166, 185, 204, 224, 244, 263, 283,
302, 322, 342, 361, 381, 399, 419, 428, 448, 468

But as Weather Vane correctly pointed out in the comments, I had constructed a new sign using only stickers (399) which is not permitted.

Edit:

The maximum distance marker I have managed to construct is

444km

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

8, 26, 46, 66, 85, 94, 114, 129, 148, 163, 183, 193, 212, 228, 248, 267, 287, 307, 326, 345, 365, 385, 404, 424, 444

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).

Original

I had originally thought I had a solution with distance

468 km

Using the following signs

5, 25, 45, 48, 67, 87, 107, 126, 146, 166, 185, 204, 224, 244, 263, 283,
302, 322, 342, 361, 381, 399, 419, 428, 448, 468

But as Weather Vane correctly pointed out in the comments, I had constructed a new sign using only stickers (399) which is not permitted.

Edit:

The maximum distance marker I have managed to construct is

442km

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

8, 26, 46, 66, 85, 94, 114, 129, 148, 164, 184, 198, 218, 227, 247, 267, 286, 305, 325, 345, 364, 383, 403, 423, 442

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).

Original

I had originally thought I had a solution with distance

468 km

Using the following signs

5, 25, 45, 48, 67, 87, 107, 126, 146, 166, 185, 204, 224, 244, 263, 283,
302, 322, 342, 361, 381, 399, 419, 428, 448, 468

But as Weather Vane correctly pointed out in the comments, I had constructed a new sign using only stickers (399) which is not permitted.

Edit:

The maximum distance marker I have managed to construct is

443km

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

8, 26, 46, 66, 85, 94, 114, 129, 148, 164, 184, 198, 212, 228, 247, 267, 287, 306, 325, 345, 365, 384, 403, 423, 443

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).

Original

I had originally thought I had a solution with distance

468 km

Using the following signs

5, 25, 45, 48, 67, 87, 107, 126, 146, 166, 185, 204, 224, 244, 263, 283,
302, 322, 342, 361, 381, 399, 419, 428, 448, 468

But as Weather Vane correctly pointed out in the comments, I had constructed a new sign using only stickers (399) which is not permitted.