Detect when a car is in the parking position

Hello guys. My name is Chris and am from Nigeria. I am a final year undergraduate of electrical engineering. Am having problem figuring out something about what am designing for my final year project.
My project is to design a circuit that prevents drivers from answering calls while the car is in motion. The alarm is supposed to be triggered only when the car is in motion but will be deactivated as soon as the car is parked. The driver must park his vehicle to deactivate the alarm.
my question is, what technique or electrical property of the car do I use to detect when it is stationary or in motion? I thought t about using the break light to know when the car is parked but I discovered that the break could be applied but it doesn't mean the car will eventually stop so I stroke it off.

I guess you could use an accelerometer (commonly used in mobile phones). The output of this can be used to enable/disable the alarm. This is a non-invasive method. In fact for a mobile phone that has an accelerometer you could write a program that can be used to detect motion - the program should be started when inside the car and stopped when outside, else it will trigger the alarm even when the person is walking ;-)

The following are invasive methods, in that they require you to make some sort of connection/interface to the vehicle's system(s).

You could somehow tap into the vehicle's diagnostic system (OBD-I/II is pretty common) and use standard OBD commands to get the vehicle's speed readout - this will (or should) directly give you the vehicle's current speed in kmph or mph.

Yet another way is to use the vehicle's odometer cable (one that mechanically relays the wheel's rotation to the instrument cluster) - if you can somehow add a mechanism to do the same thing that the vehicle's own speedometer does and get a voltage out (tacho??) then this voltage can be used to control the alarm.

Should this work in old cars as well as new cars?
Does it need to distinguish between a drivers phone and a passengers phone?

These questions need to be answered before we can give a good answer as there are multiple ways to detect if the car is moving.

Yes to both questions @ John

I think your problem is harder than a motion sensor. For that you can mount a magnetic pickup to the drive shaft if it is rear wheel or to the half shaft if it is front wheel. This will work with any year car and was how after market cruse controls worked back in the 70s. Your big problem is knowing that the driver of the car is trying to use the phone rather than a passenger. You can't even do this with pairing the phone with the driver as the driver could hand it to a passenger. If you can figure that out you have a real product to sell.