Here are the relevant d20 Modern collision rules.
Pick the fastest speed among the objects:
Alley speed (1-4 mph) d2
Street speed (5-34 mph) d4
Highway speed (35-80 mph) d8
All out (81+ mph) d12
Pick the smallest object size in the collision:
Collosal: 20 dice
Gargantuan: 16 dice
Huge: 12 dice
Large: 8 dice
Medium: 4 dice
Small: 2 dice
Tiny: 1 die
smaller than tiny: 0 dice
Apply the appropriate multiplier:
collision with stationary object: x1
collision with moving object, head-on or 45 degrees from head-on: x2
collision with moving object, perpendicular: x1
collision with moving object, from behind or 45 degrees from behind: x1/2
side-swipe: x1/4
This is the damage to the vehicle (and to whatever it hit). Occupants receive damage as follows:
vehicle provides no cover (e.g., a motorcycle): x1
vehicle provides 1/4 cover (e.g., a helicopter with cargo doors open): x1/2
vehicle provides 1/2 cover (e.g., rear bed of a pickup truck): x1/4
vehicle provides 3/4 cover or more (e.g., a typical car or truck cab): x0
occupants may make a reflex save (DC 15) for half damage
You read that correctly. Under d20 modern rules, occupants of a typical car NEVER take damage from collisions (unless the driver loses control and the vehicle rolls over or the vehicle takes enough damage to explode).
Speeds of both objects are reduced by two categories.
The driver has to make a Drive check (DC 15) to avoid losing control of the vehicle.
If the collision disables the vehicle (reduces it to 0 HP or less) AND deals damage greater than or equal to half of its full normal hit points (Most cars have HP in the 30s and hardness of 5), the car explodes in 1d6 rounds, dealing 10d6 damage to everyone within the vehicle (Reflex save (DC 20) for half), and half that much to everyone within 30 feet (Reflex save (DC 15) for half). A vehicle reduced to the negative of it's full HP total (e.g., -36, for a car with max HP of 36) is destroyed and cannot be repaired.