The other day, my brother drove his car down the street at -

The other day, my brother drove his car down the street at ____ I thought was a dangerous speed. (2求高手,给出答案,并解释at后面的结构,尤其是thought的宾语和was的主语难道就是一个关联词吗,万分感谢大家的答案似乎偏离了我要问的啊，再求高手，谢谢