An unrealistically low speed limit can actually cause more harm than good. In fact, it can actually lead to more crashes. Second, some drivers will obey the lower posted speed while others will feel that it is simply unreasonable and simply ignore it. This disrupts the uniform traffic flow and increases crash potential between the faster and slower drivers. Third, when traffic is traveling at different speeds, the number of breaks in traffic to permit safe crossing of vehicles or pedestrians is reduced.
Pedestrians have greater difficulty in judging the speed of approaching vehicles.
The basic intent of speed limits is to identify a safe and reasonable limit for a particular section of road. This reflects the safe speed as determined by a large majority of the drivers. Reducing the limit below that 85th percentile will probably be detrimental to safety.