You can say either but it is more common to say congested when speaking about traffic. Overcrowded tends to be used to refer to numbers of people in a confined space. Are you only talking about one busy road? If you mean the traffic in general, it would be more usual to say for example: The 'roads' 'were' congested which is why I am late.

Reach what?

1a. The roads were congested, so I couldn't make it on time. 2a. The highway was congested, so I couldn't make it on time. 3a. There was a lot of traffic, so I couldn't make it on time. 4a. There was a ton of traffic, so I couldn't make it on time. 5a. The traffic was bumper to bumper, so I couldn't make it on time. or 1b. I'm late because the roads were congested. 2b. I'm late because the highway was congested. 3b. I'm late because there was a lot of traffic. 4b. I'm late because there was a ton of traffic. 5b. I'm late because the traffic was bumper to bumper. I think #3b and #5b are used more than the others in my area of the United States.

"the road was congested, so I could not make it on time" Or, even better: "There was a lot of traffic, so I could not make it on time"