Why did חז"ל make so many גזירות related to carrying on Shabbos?
Daf Yomi just started the 6th פרק of Shabbos which deals extensively with גזירות related to carrying on Shabbos. The Gemara has extensive discussions about what jewelry women can wear on Shabbos, what are animals allowed to go out with, etc. all גזירות in fear of the person violating the איסור דאורייתא of carrying 4 אמות in a רשות הרבים. It is patently clear that in the the time of חז"ל that they felt that this was a very valid concern as we know that that חז"ל followed a number of rules when making גזירות, two of them being:
1. מילתא דלא שכיחא לא גזרו ביה רבנן - We don't make גזירות for far-fetched cases.
2. we don't make a גזירה לגזירה - We only make גזירות to protect people from violating an איסור דאורייתא, we don't make גזירות to protect people from violating an איסור דרבנן.
Based on the above חז"ל would only have made all of these גזירות regarding carrying if there was a real concern that people would violate the איסור דאורייתא of carrying in a רשות הרבים. What this means is that there must have been MANY places deemed רשות הרבים in existence in Israel (and Bavel) otherwise they never would have made all of these גזירות as it would have been a מילתא דלא שכיחא and/or a גזירה לגזירה.
However, this is very very difficult. תוספות on 64b already makes the claim that we don't have a רשות הרבים דאורייתא nowadays because we don't have 600,000 people and our streets aren't wide enough. This opinion of Rashi and Tosafos has been accepted להלכה and is the basis of all city עירובין. But, if this is true, then how in the time of חז"ל did they ever have a רשות הרבים דאורייתא? Why would they make all of these גזירות if they didn't? It is not believable that in the time of חז"ל the population of the cities in Israel was larger then today. The only city today, in 2012, with a population over 600,000 is Jerusalem and only a fraction of that is in the old city (which is the only part of the city that existed 2000 years ago and in fact was smaller then it is today). Every other city in Israel today has a population less then 600,000 people. There is no way that in the times of חז"ל the cities were anywhere near as large given what we know of the population etc.
In short, how could it be that in the time of חז"ל two thousand years ago they were so worried about people violating an איסור דאורייתא of הוצאה when we can't find a רשות הרבים דאורייתא today? Brooklyn, with a population of 2.5 million people (larger by far then any city 2000 years ago) is the only place that I have heard of that there even is a discussion about. There are עירובין in every large city in Israel (based on the presumption that the city is not a רשות הרבים דאורייתא) and many large cities/communities in America. How do we reconcile this with the many גזירות that חז"ל made regarding carrying on Shabbos (including canceling the miztva of Shofar when Rosh Hashana falls out on Shabbos)? How come חז"ל felt that a רשות הרבים דאורייתא existed and we don't?