When a police officer pulls over a vehicle without reasonable suspicion required for this stop to be lawful is that considered false imprisonment? Or would that require the driver also to be arrested to constitute false imprisonment?
Police have the authority to stop a vehicle if they have a reasonable suspicion of a crime. That means that there must be an articulable ground for believing that there probably was a crime, which includes a specific fact (like "seeing a trail of blood", and not "it's suspicious that an old car would be driving in this neighborhood"). Without a reasonable suspicion, the detention is not with legal authority. PC 236 states that
False imprisonment is the unlawful violation of the personal liberty of another
The stop is a violation of the personal liberty of a driver, and without lawful authority, the stop is unlawful. Although the doctrine of qualified immunity might suggest that police officers cannot be prosecuted, there is no qualified immunity for California police officers accused of false arrest or imprisonment.
Unlike some other jurisdictions, there is no requirement for an officer to have "probable cause1" or suspect an offence as the police can stop a vehicle for any reason under section 163 Road Traffic Act 1988:
(1) A person driving a mechanically propelled vehicle on a road must stop the vehicle on being required to do so by a constable in uniform or a traffic officer.
(2) A person riding a cycle on a road must stop the cycle on being required to do so by a constable in uniform or a traffic officer.
(3) If a person fails to comply with this section he is guilty of an offence.
There is no associated power to search the vehicle or its occupants but under section 164 and section 165 the driver must produce inter alia their licence, name, date of birth, address, insurance details and other relevant documents as the case may be.
Note that although vehicle stops can be random, police officers are subject to the public sector equality duty under section 149 Equality Act 2010 and not permitted to stop a vehicle solely based on the occupants' protected characteristics.
1The term "probable cause" is not used in the UK, but roughly equates to somewhere around reasonable suspicion / reasonable belief