Both have their merits, but on average, still images generally work better for generating leads.
Videos are great for engaging individuals and communicating larger amounts of information, but when it comes to getting drivers to provide contact information, still images work better. Ads will typically see a lower cost-per-lead in this scenario.
This being said, best practices call for a mix of both still images and video running simultaneously. Diversity in the ads you put out will help ensure drivers who might be swayed by one message and not another are being engaged.