So the defendent has to prove his innocence rather than the accuser proving guilt.
The burden of proof shifts back and forth depending upon the evidence. Once the prosecution has overcome your presumed innocence and established a prima facie case, establishing at least the minimum case required to convict, then it's the defense's burden to prove the prosecution wrong.
This ruling says that a trained officer's speed estimate meets the requirement for a prima facie case of speeding. Without any proof to the contrary from the defense, it's enough to convict.
If, for example, there were three witnesses in the vehicle who all testify that the vehicle's speedometer, or a GPS receiver on the dashboard, indicated a speed at or below the current speed limit at that time, then that testimony might outweigh the officer's estimate.