The story here on the La Voz de Almería site has more details of the one person per car rule.

Read More for a Google translation.

Driving in your vehicle in company already carries fines of up to 1,000 euros

The Royal Decree is modified and affects how to get around by car during the crisis

Starting this Thursday, March 19, during the state of alarm, you will only be able to drive alone. Going with a companion may carry a penalty. This is reflected in the modification that the Government has made in Royal Decree RD 463/20, as pointed out by Cadena SER.

Therefore, movements with more than one person inside the vehicle are prohibited. The agents of the authority have the order to put sanctions from 300 euros to 1,000 euros. This is stated in Royal Decree RD 463/20 modified by the Government.

The movement must be carried out individually, except when accompanying children, the elderly, dependents who have some type of disability, or a reason that is really justified.