Special Spanish license plates

While in Spain, I saw plenty of vehicles (mostly lorries, buses and taxis) sporting a small additional plate reading either “MP” or “SP”.

What’s this?

In Spain, finding MP or “Materiales Peligrosos” on a truck/lorry means that it’s transporting hazardous materials.

Ah… thank you, Ponster.

Could “SP” be something like “servicio público”?

[Ramius]… one bump only.[/Ramius]

Yes, that’s exactly what it means. All taxis and public buses have it.