The small black triangles (on the road) are displaying traffic hold ups ahead on the road you are on, the arrow points in the direction that the hold up is in.

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Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.