_the fatality rate in 2006 for motorists between 18 and 20, according to the national highway traffic safety administration, was more than double that for drivers over 70.

이 문장에서 밑줄친 that은 어떤 쓰임으로 쓰였나요 ?? 관계대며사는 아닌것 같구.....
친절하고 자세한 답변 부탁드립니다.