Hi Tammy,
Mantra on the go? mmm this would be fine if you were:
a) the passenger in the said vehicle as you mentioned for safety reasons.
Brian has made a very good point about the purpose of your Saddhana? I guess it is too easy to fall into the trap of the more mantra's we get more benefit etc...etc.
Maybe a better goal maybe to work on setting a correct motivation pre-Sadhana? From my own experience if I am going to be traffic jam I will use this time to focus on my practice...of Watching my mind...I set myself a goal of controlling my anger and thoughts to other users on the road.
Setting myself little goals and realizing not all days are the same...Some days can count on one hand and some days it feels like falling off the wagon before my day has even started .
My work requires alot of travel but also as a result lots of opportunity for not so mindful road users on the road also arggghhhh!
So how can we expect our Sadhana and prayers to have power and blessings of the Buddha's to help us if we do not make ourselves a perfect recepticle to receive blessings.
Other reflections I will use throughout my travels is to reflect on impermanence? guru devotion? kindness of friends and loved ones? Than when it comes time to do your Sadhana you have done some internal housekeeping so to speak
Also you could consider doing some preliminary prayers before your day begins? I prefer to do this when I first wake in the morning, work on the practice during the day and actaul Sadhana at the end of the day not driving and quiet time.
Hope this helps?
xo