Person A - You are late. Person B - I didn't count on the traffic. Yes it is correct. It sounds perfectly natural. Here are some variations: Person A - You are late. Person B - I'm sorry. I didn't count on the traffic. <- kind of polite (the difference is an apology before the reason Person B is late) Person A - You are late. Person B - I'm sorry. Traffic was crazy! <- This is a bit of slang. (To say that something was "crazy" it could mean that it was very busy or chaotic or unexpected.)

yes