"If you see the donkey of your enemy crouching under its burden... you
shall help him with it." (Shemos 23:5)
"If you see your friend's ox or his donkey falling... you shall stand
them up with him." (Devarim 22:4)
If someone is buckling under a load, it is a mitzvah to help him unload
it. Help him rearrange his load and get back on his way.
The Sefer HaChinuch says that we learn from this mitzvah to have
compassion on others and try to help those who are suffering. When we
have mercy on others, Hashem will have mercy on us.
The Aruch HaShulchan (siman 272) says the mitzvah applies also to a
vehicle such as a wagon. In our days, we can extend this to a car or any
other motorized vehicle.
The Mishna teaches, "Carry loads with others" (Avos 6:6). Mesilas
Yesharim explains that although this mishnah refers to emotional loads and
situations of stress, the simple meaning also includes the mitzvah of
helping others carry luggage or heavy bags of any kind.