I was on my way to the bookstore and was waiting for the green light at a crossing when a girl of about ten was knocked down by a passing car, it drove off quickly.

不是说复合句中，除大主句外when引导的时间状语从句也可有自己的小主句么，那it drove off quickly.做when a girl of about ten was knocked down by a passing car,时间状语从句的小主句，I was on my way to the bookstore and was waiting for the green light at a crossing是复合句的唯一大主句有什么不对吗？而必须把it改为which成非限定定语从句。