The law does not define travelling. It creates a presumption of travelling.
Perviously, if you were found with a pistol in your car, you could be arrested, and it would be up to you to prove that you were travelling. Basically, the only way for you to not be convicted would be for you to take the stand and testify in your own defense.
Now, assuming that you meet the prerequisites, you are presumed to be travelling. This means that if you are arrested for having a pistol in your car, you don't have to testify to defend yourself at trial. It's up to the prosecutor to prove that you aren't travelling.
Whether or not you are arrested if you are found with a pistol in your car is to some extent at the discretion of the police officers involved. If he thinks that you aren't travelling (for example, if he asks where you are going and you respond "to the store to get eggs"), expect to be arrested.
For more information, you can find about 5 more threads on this on the back pages of this forum.