The radio picks up the traffic news from what's called EON - extended other networks - which generally means various BBC local stations or independents that belong to the same owning company. As well as EON, the radio also has something called AF -- auto-find. This is how it can 'follow' a national station like Radio 1 on different frequencies as you move around the country.
Both these features are controlled by the option to increase or reduce the sensitivity of the radio to weaker signals. So, if you set it to receive weak/distant signals, you will find it feeds traffic info from further-flung places. Set it to only tune to more local/strong signals and you should only hear more local traffic news. Flip side of this is that it then tends to be less able to keep following national radio if, for example, you go into a valley. How to alter that sensitivity setting varies between radios -- on the original Fiat one, it's easy to find from the menu.
[later - just realised: you can force it only to give you traffic news from the one station you tuned in to by turning off the 'EON' option - again, its in a menu somewhere]