Regional Jet 200 ER/LR (CRJ200)
Flight Crew Air Transport Pilot (ATP)
Workload; Communication Breakdown; Time Pressure
ATC Issue All Types; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert
Procedure; Aircraft; Human Factors
When we checked on with...approach while on the...arrival; we were assigned an airspeed of 250 kts or better.  We maintained 290 kts until descending below 10000 feet at which time we maintained 250 kts.  [Airport] was landing to the north.  ATC turned us to a heading of 090 which was going to set us up for about a 15 NM final for landing on the ILS 36R.  While on the base leg of the approach; we began slowing to 180 kts in order to begin configuring for landing.  ATC questioned what speed we were flying.  I reported 180 kts and the controller informed us that we should not have slowed without telling him that we were slowing down.  I questioned if we had an assigned speed to which he responded that we were expected to maintain 250 kts until told to slow.Being turned on base for a 15 NM final; it is necessary to begin slowing down in order to configure the airplane and be stable by 1000 feett.  Since [ATC] normally has us slowed back to 170 or slower by this point and it was necessary to begin slowing in order to land the airplane safely; I did not even consider that the...approach controller would still want us at 250 kts as we approached the turn onto the localizer in IMC.  Technically; ATC was correct that we were assigned 250 kts or better when we checked on over [fix] and that slowing without telling them was a violation of that instruction.  However; expecting a CRJ200 to be at 250 kts while intercepting a localizer in IMC on a 15 NM final is not a realistic expectation and would have likely resulted in a go around in IMC due to not being able to be stabilized by 1000 feet.
CRJ-200 First Officer reported slowing aircraft to 180 KTS to configure for landing despite ATC instruction to maintain 250 KTS. ATC advised that pilots should have informed ATC of speed reduction.