“It’s partly due to traffic, but the main reason you pour concrete in the extreme early hours is due to temperature.
Concrete is actually a substance undergoing a chemical reaction and continues to get warmer.
In the south, it’s not uncommon to pour concrete in the early morning hours before it gets too hot. If the concrete gets too warm it becomes brittle really fast. There are ways of dealing with heat such as adding ice to the mix, but it is not the preferred way to go.
This is also the reason why in west Texas you don’t see to many concrete roads. It’s really hard to maintain the temperature during a large placement.
So yes, there is some concern with traffic, but it has to do more with heat.
Now, some of you may say that the slab of your house was placed in the heat of the day. Yes, but the strength lost in the heat for your house is not enough to effect the overall structure. Bridges, highways, highrises, and large placements where high loads are experienced can get susceptible to the heat and weaken the overall structure. . . .” [kjb434, commenting on About Those “Early Morning” Concrete Pours]