Hey, look, it’s another system for Dirge inspired by State of Decay. This one is probably reinventing the wheel a little bit, but I don’t have any other D20 systems for cars and I wanted to create something custom tailored to present the simplest possible way to resolve running over zombies. The system is tuned to make hitting enemies with your car both highly effective and highly dangerous to the car; anyone that’s hit a deer knows that the car does not roll away unscathed in a collision with a soft body, but you still might want to do it rather than risking your delicate human flesh against the claws and teeth of the undead.
I tried to build the systems to be at least not totally inconsistent with the realities of such collisions, but I am not an automotive safety engineer, and erred on the side of, “yes, you should totally swerve to hit that zombie.” Anyone that is more familiar with automotive impacts is welcome to let me know what my intuition got wrong.
Note that this system is designed for Dirge, where the vast majority of people and zombies have HP in the 10-20 range and exceeding 30 is extremely rare. The numbers might stop making sense pretty quickly if you’re using a more traditional D20 build where HP could hit well over 30 while still being expected to be within the human norm.
Building an Automobile
Each vehicle is a somewhat abstract collection of several statistics:
- Core Statistics:
- Str (Power): A car’s Power represents raw horsepower. It primarily controls how fast the car can accelerate, but would also be used to pull or push great weights.
- Dex (Handling): A car’s Handling describes how agile it is. It assists most driving rolls, representing speed at turning and reacting immediately to gas and brakes.
- Con (Structure): A car’s Structure describes how ruggedly built it is. It provides a vehicle’s base HP and is used to keep the vehicle running after major damage.
- Weight: A car’s Weight describes its structural mass relative to other vehicles. It increases HP, DR, and damage, but is a major penalty to acceleration. For purposes of damage and acceleration only (not for HP or DR), increase the vehicle’s weight by +1 for every ton of cargo (rounded down); a bed full of bricks slows your truck considerably, but also increases the mass you can deliver to a target.
- Derived Statistics: All these stats are adjusted any time a core statistic changes (even temporarily).
- Hit Points: HP are equal to Con + (Weight x 5) (e.g., a Con 14, Weight 4 truck has 34 HP). This primarily represents the engine, suspension, and other components vital to keeping the car moving.
- Damage Resistance: DR is equal to Con + Weight – 2 (e.g., the truck in the previous example has DR 16). Any damage that doesn’t bypass the vehicle’s DR may have cosmetic effects, but doesn’t impede the functioning of the vehicle. Called shots from guns might be able to hit the tires, driver, or undercarriage at the GM’s option, bypassing some or all DR.
- Damage: Damage starts at d4 and increases by one step per point of Weight (e.g, the truck above has d12 damage). This represents the mass that the vehicle can put into a target, and is multiplied by current speed as described below. Beyond d12, increase dice steps by your preferred method.
- Base Acceleration: A vehicle’s base acceleration is equal to Strength – (Weight x 2) (e.g., the example truck, at Str 14, has a base acceleration of 6, slower than a tiny Str 8, Weight 0 compact).
Like the scores they’re based on, Power, Handling, and Structure have a value and a modifier, using the same system as character ability scores (e.g., Power 14 has a modifier of +2). For ease of reference, they’re listed by their original scores in all examples below.
There are several basic types of automobile in common use:
- Small Compact Car: Str 8, Dex 12, Con 8, Wt 0
- Coupe/Small Sedan: Str 8, Dex 12, Con 10, Wt 1
- Large Sedan/Wagon: Str 10, Dex 12, Con 10, Wt 2
- Small SUV, Truck, or Van: Str 12, Dex 10, Con 12, Wt 3
- Full Sized SUV, Truck, or Van: Str 14, Dex 8, Con 14, Wt 4
Apply the following modifiers based on the specific type of vehicle (generally representing pricier upgrades than the standard):
- Sporty: +4 Str, +2 Dex
- Rugged: +2 Str, +4 Con
- American Performance: +2 Str
- European Performance: +2 Dex
- All Steel Construction: +2 Con, +1 Weight, No Crumple DR
- Four Wheel Drive: Halve penalty to drive on uneven or slippery terrain
- Launch Control: Driving +4 when accelerating from a stop
- Carbon Fiber: Weight -1, Dex +2, Con -2
So, for example:
- Expensive Small European Sports Car: Str 12, Dex 18, Con 8, Wt 0; Four Wheel Drive, Launch Control; HP 8, DR 6, Damage d4, Acceleration 12
- Modern American Muscle Car: Str 16, Dex 14, Con 10, Wt 2; Launch Control; HP 20, DR 10, Damage d8, Acceleration 12
- Big American Hemi-Powered Work Pickup: Str 18, Dex 8, Con 18, Wt 4; Four Wheel Drive; HP 38, DR 20, Damage d12, Acceleration 10
- 1970s Wood-Paneled Station Wagon: Str 10, Dex 12, Con 12, Wt 3; No Crumple DR; HP 27, DR 13, Damage d10, Acceleration 4
Player characters may want to custom modify their long-term cars for the apocalypse. Options include:
- Armor Plating: Add +1 Weight (and all HP, DR, and Damage mods that entails), Add Weight in additional DR vs. guns and explosions, Crumple DR -6
- Disabled Airbags: Lose Airbag DR, but also don’t worry about them deploying when you’re mowing down zombies
- Reinforced Windows: +3 Hardness to windows, but you can’t effectively get through the grating to reach things, escape the car, or shoot any direction but perpendicular to the car
- Reinforced Bumper: +2 DR when hitting something with the front of the car, Crumple DR -4
When you’re driving along and you see something you’d like to swerve to hit, follow these steps:
- Establish how fast you’re going in Miles Per Hour (using the acceleration rules, below, if you aren’t at cruising speed but see the target some distance away).
- Divide speed by 10 (minimum 1); roll that many damage dice (e.g., 40 MPH in a Weight 4 vehicle does 4d12 damage).
- All targets make Reflex/Acrobatics checks for half damage vs. a DC of the driver’s Driving (including car’s Dex modifier) + 10. This save might be for zero damage if there is an immobile structure within arm’s reach that the driver won’t plow through to get you. Zombies are usually too stupid to bother to save.
- If the target saved, deal damage representing being clipped by the car and/or bouncing hard off the ground. If the target did not save, the target takes full damage and, if living, must make a Fortitude/Stamina check, DC equal to the damage taken, to avoid becoming Fatigued*.
- Deal damage to the vehicle equal to the damage dealt to the target**. If there are multiple targets, apply the car’s DR separately against each hit. Apply damage to the vehicle equal to whatever gets past (see below).
- Total the damage that the car just took and apply it to the passengers. Apply up to three times the car’s DR for each passenger (once for the car’s Crumple DR, once if the car has airbags, and once if the passenger is wearing a seatbelt). If the Crumple DR reduces the damage to zero, the airbags do not deploy (so armor plating and a reinforced bumper can make this more likely). A deployed airbag provides a -6 penalty to Driving for the next two rounds if you want to keep going after the collision, and -2 ongoing until you can detach the deflated bag.
- Make a Driving check (including the car’s Dex modifier as an additional bonus to your roll). The DC is equal to the damage multiplier you used (e.g., 2 for 20 MPH) + terrain modifier (0 for open interstate or empty flat ground, 5 for open highway or other wide street, 10 if the street is cluttered or if it’s clear but narrow, 15 if it’s a cluttered narrow street or an alley, and up to an additional +6 for slippery or uneven terrain).
- If you succeed, you can keep going (assuming the car’s still intact) with no loss in speed or direction.
- If you failed by less than five, you have to brake to avoid spinning out or crashing into an obstacle, and come to a dead stop.
- If you failed by five or more, you collided with something, the target got caught in the undercarriage, or something else destructive. Come to a dead stop and take additional damage as if you had hit something stationary (see below).
* This is a nod to the ongoing trauma of being hit by an automobile. You can go grittier with this if desired, imposing long-term breaks or internal bleeding, but only if your game is meant to be incredibly lethal (survivors are unlikely to have access to a hospital to fix such trauma).
** At extremely high damage points, the max damage a target can do to a vehicle is its maximum HP plus size modifier (0 Fine, 5 Tiny, 10 Small, 20 Medium, 30 Large); hitting a possum, even at highway speeds, is probably not going to do much to most vehicles, but hitting a deer certainly will.
If something you’d rather not hit appears in front of you at full speed, make a Driving check (+ car’s Dex modifier) at the same DC as step 7 above (Speed/10 + Modifiers).
- If you succeed, you missed the target.
- If you fail, you couldn’t quite turn in time.
- A mobile target’s Reflex/Acrobatics DC is only 10, but it’s otherwise treated as a normal hit.
- If the structure was immobile:
- Deal damage normally, tracking the target’s hardness and HP if there’s any chance at all you could knock it down or break through.
- Deal twice the damage the target took to break (or all the damage you dealt if it didn’t break) to the vehicle.
- For example, a truck slams into a thin wooden wall at 60 MPH deals 6d12 to the wall for 40 damage; the GM rules it has Hardness 5 and 20 HP to break enough to let the truck through. The truck spent 25 damage on the wall so takes 50 damage itself as it plows through the wall. If the passengers were wearing their seatbelts and haven’t disabled the airbags, they’re probably fine… if not, things might have just started to go very badly for them.
Whenever a vehicle takes damage:
- Make a check using the vehicle’s Con modifier against a DC equal to the damage that the car just took.
- If the roll succeeds, the damage is structural but hasn’t broken anything vital yet.
- If the roll fails, the car has a bent wheel well, misaligned suspension, cracked hose, or some other minor problem. Reduce Str or Dex by 2 until repaired (this can be temporarily patched in a few minutes once stopped, and is fully repaired once the car is restored to full HP).
- If the vehicle is reduced to half HP or less (and then every additional time it takes damage under half HP), make a check using the vehicle’s Con modifier against DC 12.
- If the roll succeeds, the car is obviously messed up but somehow still functioning.
- If the roll fails, something major has happened to the engine and it ceases functioning. A Mechanic check at DC 20 (roll once per half hour), DC 25 (roll once per minute), or DC 30 (roll once per round) can get it going again until it takes more damage or is turned off.
- If the vehicle is reduce to 0 HP, it is totaled.
Each mechanic working on a vehicle can restore the Mechanic skill total in HP per day to a damaged vehicle. This rate is halved without a fully stocked and powered body shop (and may be reduced to 0 without any level of sufficient tools). If the vehicle stopped working due to falling below half HP, it will likely require several replacement parts before it can be fixed. If the vehicle was totaled, it will require extensive new parts if it can be fixed at all (GM’s option).
Other Driving Challenges
From a dead stop, the driver of the car makes a Driving check (adding the car’s Dex modifier and the bonus from if it has Launch Control). Divide the result by five and multiply by the car’s base acceleration to see how fast it got to in MPH this round (e.g., A roll of 16 on a car with Acceleration 8 means the car is going 24 MPH at the end of the first round).
When already moving, a vehicle can accelerate up to four times its base acceleration in MPH each round (to the maximum of its top speed). For example, on the second round, the car above can add up to 32 more MPH, so could be going 56 MPH by the end of the second round.
The space in feet that took up is [MPH at the end of round 1] x 4.5* (e.g., if you make it to 60 from a dead stop, that took 270 feet). If you don’t care to do the math, really most of the time it will only matter for post-apocalyptic drag races and seeing if one car can catch another on an open stretch of road.
Driving backwards, if Burn Notice is to be believed, prevents airbags from going off on an impact. There’s also usually less vital structure in the back. So when you’re going backwards:
- Increase the car’s DR for impacts by 4.
- The airbags won’t trigger.
- Acceleration is halved and the car probably has a much lower top speed.
- All Driving checks are made at -6.
A Bootlegger’s Reverse (switch from reverse to forward without losing significant speed) is DC 10 + 1 per 10 MPH (don’t forget the -6 driving backwards penalty).
Mapping and Short Accelerations
For every 10 MPH, a car goes 90 feet in a typical six second combat round. This makes using cars on a tactical map largely impractical, and so the rules don’t pay much attention to maneuvering on a grid.
If you want to just quickly start a car and run it into someone in a tactical situation, assume that the car can get up to enough speed to do one die of damage within about 20 feet and two dice within 30 feet. It’s left up to GM’s discretion what happens if that means a zombie is now pinned to a wall.
If you absolutely need to know how fast you’re going within a fixed distance to accelerate, the (simplified as well as my retained algebra allows) formula is: MPH = [MPH at the end of round 1] x 1/6 x square root of ([feet available to accelerate] x 8 / [MPH at the end of round 1])*.
Unless modified with a reinforced grill, a standard car window has Hardness 3, Break HP 6, and windshields have twice that many HP. Essentially, unarmed zombies dealing 1d4 should beat on it for a while, slowly cracking it as the survivors inside frantically try to figure out how to get out of this situation, while someone armed with any kind of weapon should have no difficulty breaking through.
In a pure straightaway, chases come down to distance, acceleration, and top speed. But how many uncluttered roads are there in the zombie apocalypse? If you’re trying to chase down a rival band of survivors in a car (or get away from a horde of cheater zombies that can go over obstacles you have to drive around), use your chase mechanic of choice. Something simple with the lead driver setting a DC with a Driving check and the pursuers trying to beat it (failure on either part closing or opening the lead or, if by five or more, resulting in a dramatic spinout into a horde of zombies) should also suffice.
*All conversions from MPH to FPS are using an approximate 1.5 multiplier instead of the more accurate 1.4667.