There are other devices on the bus looking for messages specifically from the throttle address. The idea is that you want to change those messages, but only sometimes (i.e. when cruise is on), so MITM'ing the messages is the easiest way to implement this.

Ah okay, I assume you can't just spoof the address then, thanks.

You could, but you'd have two devices reporting different values for the same address and the devices reading that value wouldn't know what to do.

I don't own a car so can't stick my head under the driver's seat to verify, but it seemed like many recent cars has:

- Reporting features for brake, gas, steering inputs, a la SNMP

- Computer assisted brake, with a backup linkage

- Fully electronic gas pedal, with a speed sensing cruise control

- Electric/electronic power steering, using a torque/relative deflection sensor

These features are case-by-case:

- Command based "write" access for brake/gas/steering

- Absolute steering position sensor

So a lot of older cars require special spliced cables to fake analog readouts or nonstandard digital links to retrofit a self drive, and newer cars still needs to be purchased with those input features enabled as well.