I would personally say: "Improve traffic conditions" and "Improve environmental conditions". But what you said above does technically make sense. Improve can mean similar to "ease" and "solve" means the problem has completely ended. (which in the case of traffic is not likely). Ease: To free from pain, worry, or agitation: Improve: To raise to a more desirable or more excellent quality or condition; make better. Solve: To find a solution to. For example: Traffic conditions have eased since the new road was built. or: Traffic conditions have improved since the new road was built.