"我们过去了" was he joking?
Probably more natural to be 「我们过去啦！」, which means "Let's go across [the intersection]!"

Reasoning:


啦 can be mistaken as 了 in this context, especially when the stress is on 过去 so that the 啦 is pronounced through quick.
Your friend was smiling, which may imply a positive mood, and the imperative with 啦 can serve as an appropriate expression here.