explosions can occur in that situation but actually often do not. You need a lot of gas vapors in the immediate air, and that generally does not happen. The explosions tend to happen a few minutes after, if a small fire starts, like say from some fuel leaked from the fuel line hitting the engine on collision (not enough fuel vapor to explode) and the fire causes the gas tank to heat up and then explode from over-pressure (boiling of the gas).
Instantaneous explosion is uncommon. It can happen, but usually does not.
If you crack the gas tank wide open so fuel spills out, then the fuel pool will ignite and that can be somewhat explosive but is usually more of a whoosh (like when you start a propane barbecue). Explosions mostly need confined space to force overpressurization, and that means heating up the tank until it blows from pressure and THEN the fuel will go boom as soon as it gets access to oxygen.