You think there is only 1 case. In fact, there are 2.
The first one is the HKSAR Government prosecutes the driver of the rear vehicle. Your testimony is usually not required if the police on-scene has determined that there is enough evidence to charge careless driving. It does not matter to you regardless of its outcome.
The second one is your civil case against the driver and the owner of the rear vehicle, which you have to do it yourself.
The HKSAR Government will not help you to recover your loss.
2015-07-30 09:47:18 補充:
Sorry about making the mistake.
But the answer is still correct:
1. The police does not require you to testify. If there is enough evidence, the police can still charge you.
Also - you can remain silence at all times.
2015-07-30 09:48:27 補充:
2. A civil case for damages is a separate case.