改变一下说法：
The traffic police believe （he）has caused the car accident.
如果把这句改为定语，该用who还是whom取代 he 呢？

呃，楼上说错了吧，正确的是That is the driver who has caused the car accident.（the traffic police believe）作为插入语。“I believe”“he thinks”这类表达作为插入语应该很常见吧

who，police believe是插入语。

That is the driver who the traffic police believe has caused the car accident

这是个双层嵌套从句。
整个从句是 the driver 的定从，定从的主语是 the traffic police，谓语是 believe。
这个定从中又包含了一个宾从 who has caused the car accident。这个宾从在定从中作宾语，has caused 是这个宾从的谓语。

二楼是对的，关系代词在定语从句中的宾语从句中充当主语

这里不是插入语，用who

好一个套娃在后面的宾从中做主语那就只能选who

难道不是用that?the traffic police believe driv has caused the car accident.