Usually, you use "overcrowded" when talking about people. The room was overcrowded (with people), so it was a fire hazard. You can use the word "congested" more naturally with traffic. The road was congested, so I couldn't reach on time. (incorrect) The road was congested, so I couldn't get here on time. (good) The road was congested, so I couldn't reach you on time. (good) The road was overcrowded, so I couldn't get here on time. (ok, but "congested" is much better) You can also say: There was a traffic jam, so I couldn't get here on time.