In addition to what seatrout has usefully provided, you may like to know that an offence of careless driving is made out when it is shown that the standard of driving �falls below that expected of a careful and competent driver�.
It is not necessary for a police officer to witness the driving as other witnesses can be called. Further, it is not always necessary for the driving itself to be witnessed at all. The offence can be proved by showing that that an accident was caused because of careless driving when only the results may be evident. Colliding with the rear of a stationary vehicle is a classic example of this. Nobody needs to see the driving that led up to the collision. The fact that the collision occurred would often be sufficient to prove the offence.
A few years ago a driver drove through a barrier and down an embankment to come to rest on a railway line. A train collided with his vehicle with horrendous results and, I think, ten deaths. He was convicted of causing death by dangerous driving, although no evidence was provided concerning the manner of his driving as nobody witnessed it. The results were sufficient for a conviction.