下来 in these sentences means ALREADY DONE SOMETHING like the perfect tense in English. Translation: The travel plan has been made. The car in the front has stopped I have taken down his telephone number.

Indicates continuation from the past towards now 定下来了 = has been finalised It suggests plans were begun in the past and have coninued up until now. 了 indicates they have been finalised. 记下来 = remember In thhis case suggests remebering it up until now

they are parts of one word 旅行 计划 已经 定下来了。 == 定下来了 = arranged 前面的 汽车 停下来了。 == 停下来了 = stopped 我 把 他的 电话 号码 记下来。 == 记下来 = wrote down