The victims, all from the same family, were returning to Ajuza village in Patan District of Gujarat when the mishap took place in the hilly area near Ambaji.
Fifteen people, including seven women and five children, died on the spot, while three died at the Palanpur Civil Hospital.
Of the injured, eight persons were being shifted to Ahmedabad because of their critical condition.
Chief Minister Narendra Modi expressed a deep shock over the tragedy, and announced Rs 1 lakh as immediate relief to the family-members of each deceased from Chief Minister's relief fund.