said by Dodge:You are right. I just checked with the realtor and they said that there is an implied 20' in front. So on the house that has a 20' in the back it also has implied 20' in the front, effectively removing 40' from the property.
I wouldn't imply that, at all. The street easement isn't likely to ever be an issue, it's there in case the city needs to work on the pavement or, possibly, a buried sewer, water or gas line. Possible, but the odds are very small.
The rear drainage easement isn't likely to ever be an issue, either. The fact that it's 20' wide almost implies that the pipe is entirely inside your property (easements often are center +/- 10') so a fence at the back of your property wouldn't even be an issue.
All in all I'd put those easement concerns way, way down on my concerns list.