SOME OTHER RULES AND REGULATIONS

Aboo Hurayrah radhiyallahu anhu relates that the Apostle of Allah sallallahu alayhi wasallam said,
"The younger person should salute the older person; he who is passing on (the road) should salute those who are sitting; and (men of) the smaller group should salute (men of) the larger group."

Bukhaari

In another narrative of Aboo Hurayrah radhiyallahu anhu, it is stated that
"the traveller on horseback (or any other carriage) should salute him who is going on foot."

COMMENTARY:
When two persons meet, the one who is younger in age should take the lead in making the salutation. In the same way, if a person who is going somewhere happens to pass by someone who is sitting, he should salute him first, and when two groups of men come across each other, men of the smaller group should be the first to greet men of the larger group, and in case a person is riding, he should, first, salute the person who is going on foot.
The wisdom of the last instruction is self-evident. The man who is traveling on horseback or in a car etc., apparently, enjoys superiority over him who is walking on foot, and, therefore, he should show humility by greeting the pedestrian first.

Ali Ibne Abi Taalib radhiyallahu anhu relates that the Apostle of Allah sallallahu alayhi wasallam said,

"If anyone belonging to a group that is passing by salutes, it will suffice for the whole group, and if anyone belonging to the group that is sitting acknowledges, it will suffice for the whole group."

Bayhaqi

WHEN NOT TO SALUTE