atChange - The default. One sign stands where the value changes and none after it, which is
where the road posts its own. Sections of a sign-posting type tile the route end to end, and no
two consecutive ones carry the same value, so every section boundary is a change.along - The sign repeats down the stretch at a fixed screen spacing, so a reader zoomed into
the middle of a long one still sees the value in force. A stretch too short on screen to fit a
sign draws none.A stretch can run 50 km. So under atChange, a reader zoomed in past its start sees no sign
until the next change, as on the road itself.
Where a section's signs are posted along the stretch they belong to.