我想問街頭問卷調查是用了甚麼sampling呢？

100%不是simple random sampling。Elements of sampling theory / Vic Barnett 已經指出了這一點。例如一個調查員在下午3時於旺角做街頭問卷調查。則上班中的人他便訪問不到。而且該調查員亦好可能選人問。其實街頭問卷調查是用了quota sampling。即是要調查員訪問一定quota的人﹐至於如何選擇受訪者則任由調查員自己決定。
For example interviewers might be tempted to interview those people in the street who look most helpful, or may choose to use accidental sampling to question those which are closest to them, for time-keeping sake.

I think it is simple random sampling.

It is not likely to be systematic sampling, stratified sampling nor cluster sampling.