TRAFFIC JAM

When there is a lot of traffic, we say the traffic is heavy. Yes, a bit strange indeed since we do not weigh traffic. An idiomatic and commonly used expression for this is a "traffic jam." 

When something is jammed, it is stuck. For example, paper in a printer can get jammed. So, when there is a lot of traffic, you get stuck in the traffic jam. 

This usually happens during rush hour, which is the time of the day when people are commuting from home to work and vice versa. 

When was the last time you got stuck in a traffic jam?

IT DAWNED ON ME

If something "dawns on" you, it means that you suddenly understood or realized something.

Dawn is sunrise, the first appearance of sunlight in the morning. So you suddenly see the light, or rather the truth or situation clearly.

For example: Just after I left my apartment, it dawned on me that I locked my keys inside.

BITE OFF MORE THAN YOU CAN CHEW

If you bite off more than you can chew, you are trying to do more than you are able to.

Example: Sam registered for 5 classes this semester. On top of that she is also working part-time at the bookstore and signed up for the chess club. She is beginning to realize that she bit off more than she can chew.

CALL THE SHOTS

Calling the shots means making the decisions or being in charge.

Example: Hey, it's your party. You can invite whomever you want. You're calling the shots.

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