På dansk Afstandsmærker
Countdown markers to a level crossing: the crossing is close, take your foot off the accelerator.
A Triangular with a red border: something ahead needs your attention. Outside built-up areas they normally stand 150-250 m before the hazard, closer inside town.
Drawing from Wikimedia Commons, free of copyright under section 9 of the Danish copyright act. Code and name follow executive order 425 of 13 April 2023 on road marking. Study material, not the legal text.