If I see a lot of pieces of bread on the side walk do I have to pick them up because bread is not allowed to be on the floor?
It is not that it isn’t allowed to be on the floor, rather because people might step on it, and it suffices to move it to the side where people will not step on it, and cause bizayon ochlin.
Additionally, if it is in a place that it will not get stepped on you don’t have to worry about it.
If it is less than a kzayis, although you should not step on it, you don’t have to pick it up.
Additionally, according to R’ Eliyashiv, food lying in the street, although we may not step on it, however since nowadays is not going to be eaten by people, does not have to be picked up since it is not considered edible.
Mishna Berura 171-11, Piskei Teshuvos 171-7. Regarding moving the food to the side, see Eitz hasadeh 29 ftnt. 4. Regarding if it less than a k’zayis see Eitz Hasadeh chapter 29 ftnt. 8 in the name of R’ Chaim Kanievsky shlit”a, R’ Schienberg zt”l and Dibros Moshe (B”M 31-15).
Regarding food found in the street, see Eitz Hasadeh 29 -4, quoting R’ Eliyashiv zt”l. (R’ N. Karelitz in Chut Shani -Rosh Hashana pg. 399 however holds that it still has to be picked up.)