Regional Jet 200 ER/LR (CRJ200)
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Time Pressure
ATC Issue All Types; Inflight Event / Encounter Wake Vortex Encounter
Flight Crew Requested ATC Assistance / Clarification
Environment - Non Weather Related; Procedure
We were cleared to lineup and wait on [Runway] 21L behind a departing 757 with landing traffic at a 4 mile final.  As the 757 rotated; we were given our takeoff clearance.  I told the First Officer that was very close to the 757 so we needed to be prepared for potential wake turbulence.  Sure enough; at 200 feet; we hit moderate wake turbulence.  Thankfully it wasn't too strong as 200 feet does not really give us too much room to work with if an unusual attitude recovery would be required.  We mentioned to the Controller that we had a good amount of wake turbulence as he handed us off (he said sorry).  I'm not sure exactly what the ATC rules are but I thought I remember hearing 5 mile spacing behind a 757; either way it would be nice to have more than just the runway length of spacing.  Looking back I should have just declined the lineup and wait knowing there was traffic on a 4 mile final with a 757 going out in front of us.  I didn't do it because I didn't want to mess up there flow/traffic lineup that they had; so I felt pressured to take it as given.  Overall the situation was manageable but I could see how it could have gone a lot worse. ATC did not give us enough spacing behind the 757 and rushed us to get out. Strict separation requirements after bigger aircraft departures.
CRJ-200 Captain reported encountering wake turbulence from a preceding B757 departing DTW; and questioned the spacing.