wouldn't be congested / subway passenger guiding

When you take the subway you'll find out：Some of the carriages is very crowded,But there are always some carriages very few people in.
Why?In fact,At the begining people random choose to enter carriage.And then,The number of queues will be as a reference.But people don't know the coming train's carriage which one is crowed and which one uncrowded.The result is crowed more crowed and also low transport efficiency; But "wouldn't be congested" can change this situation:every gate have one signal lamp，just Red、Yellow and Green,Color transformation according to the number of coming train passengers.Passengers queue according to the signal lamp.