I vaguely remember reading somewhere--I can't remember where--that a vehicle's acceleration ratings are cumulative. So, you can increase your total speed each action phase until you reach the vehicle's top speed. That leaves me with several questions about vehicle movement, though.

At what point is a vehicle considered to be "walking"?

At what point is a vehicle considered to be "running"?

How quickly can a vehicle stop? If you are in a Eurocar Westwind 3K going at 240 meters per turn, you certainly cannot drop to 0 speed in a single Action Phase, for example.

Do passengers firing out of the windows suffer the vehicle's modifiers due to movement (-1 for "walking"--from Arsenal--or -2 for "running")?

First of all, vehicle engagement speeds are a really not as important as the pilots skill and the handling in running vehicle encounters.
Walk/Run speeds affect combat modifiers.
Walk/Run speeds are acceleration modifiers, not speeds. See p. 159 SR4

So given a vehicle with walk 10/Run 25 max Speed 240

Starting at 0
Move/Speed/Mod
10/10/use walking speed mod when shooting out window
25/35/use running modifier
10/45/Use walking mods

To slow down, use Acceleration in reverse . Though a eurocar could get to zero by crashing into a wall.

How about if you accelerate 0 (i.e., maintain your current speed)? By your example, the implication is that you would suffer no penalty to shoot, but that seems odd to me. The vehicle is still moving, and in the case of the Eurocar, possibly at a very high rate of speed.

As for deceleration, I would think that a vehicle could decelerate faster than it can accelerate--perhaps at least twice as fast.

1: This is my understanding of it. You can actually accelerate past the vehicle's speed rating but the GM is supposed to start adding major penalties to handling and anything else he thinks necessissary, like repeated crash checks. The speed rating seems to represent a vehicle's safe maximum, not it's absolute maximum.

2: I'm not positive on the RAW, but my group has played it as walking speed is less than or equal to the vehicle's acceleration stat.

3: Anything over it's walking rate.

4: I've never seen this addressed in any of the books so just use something like 2xAccel for a vehicle's deceleration value. It's simple, universally applicable and reasonably close to reality.

5: I would think that these penalties are covered under the 'Attacker in a Moving Vehicle' penalty, BBB 140, in the case of hand held weapons. Mounted weapons would use the walking/running penalties of -1/-2 IMO.

I always like the clean, unambiguous and easy to remember rules of SR4. Don't you?

That's true. Repeated crash checks sounds reasonable to me.



While I agree with the basic concept, I think using the acceleration rates as the cut off values are a bit low. Using the Eurocar as an example again, that means a Eurocar is considered to be walking at speed 20 or less, and running at 21+. Considering its top recommended speed of 240, that seems rather harsh.

Maybe I'll go with something such as half or less of the top recommended speed is walking, and anything higher is running. That might be a bit generous, but I want to keep it simple.



I assume you meant "running" rate.



I was thinking something similar would work. I'll go with the 2 x acceleration for deceleration.



Good point about the Attacker in a Moving Vehicle. I forgot about that.

Correct, there is no mod based on th vehicle movement, but that does not stop you from applying other modifier as you see fit. A vehicle moving at 240 should involve some range modifiers for example.

The walk/run modifiers are only one of many modifiers you can choose to add. They are not all modifiers, and they are not the only modifiers.

I'd just ask for a crash test for a sudden stop.

You could also use a handling test to add a bonus to the stopping distance.

See p. 159
"Each hit on the test adds 5
meters to the vehicle’s movement rate."

Keep in mind Speed and Acceleration are different things. Walk/Run are acceleration mods, which reflect the change in your speed. Speed by itself has no modifiers.