Learn Spanish (Spain): Avanzamos a la mitad de velocidad por la lluvia. - We advance at half the speed because of the rain.

When using a fractional noun like la mitad (the half), un tercio (a third), or un cuarto (a quarter) before another noun, you must connect them with the word de. It is exactly like saying "the half of" in English.

Could I say la mitad de la velocidad instead?

Yes, you can! Saying la mitad de la velocidad is very common and means "half of the speed." Dropping the second la, as in this sentence, acts more like a set phrase that translates to "at half speed." Both sound perfectly natural in Spain.

Why is it por la lluvia instead of para la lluvia?

We use por here because it explains the cause or reason for the slow pace (because of the rain). Remember that por points backward to the cause of an action, while para points forward to a goal or destination.

Why does the phrase start with a (a la mitad de...)?

The preposition a is used in Spanish to indicate the rate, pace, or speed at which something happens. Just as you would say you drive a cien kilómetros por hora (at 100 kilometers per hour), you use a here to say you are advancing "at" half speed.