By the 1940s, it was apparent that if certain relationships between land use and travel could be measured, these relationships could be used as a means to project future travel. It remained for the development of the computer, with its ability to process large masses of data from these surveys, to permit estimation of these relationships between travel, land use, and other factors.

By the 1940s, it was apparent that if certain relationships between land use and travel could be measured, these relationships could be used as a means to project future travel.
①1940年代までには、土地利用と交通との（間にある）特定の関連性が測定（評価、分析）できれば、それ（ら関係性）を将来の交通需要を予測する手段として用いられることが明らかになっていた。
※It ～ that～形式主語構文：：itが明らかになっていた。→itってなに？。→itは、that以下。→that以下が明らかになっていた。)

It remained for the development of the computer, with its ability to process large masses of data from these surveys, to permit estimation of these relationships between travel, land use, and other factors.
②この（交通、土地利用、他の要因との）関連性を推定するためには、（調査から得られた）大量の調査データを処理可能なコンピュータの開発が必要であった。
※It ～to～形式主語構文

The first major test using this approach to develop future highway plans was during the early 1950s in San Juan, Puerto Rico, and in Detroit (Silver and Stowers, 1964; Detroit Metropolitan Area Traffic Study, 1955/1956).

The first major test using this approach to develop future highway plans was during the early 1950s in San Juan, Puerto Rico, and in Detroit (Silver and Stowers, 1964; Detroit Metropolitan Area Traffic Study, 1955/1956). 
③将来の道路計画を策定するために、このアプローチが用いられた最初の本格的な試しは、1950年代初期にサンフアン(San Juan)、プエルトリコ(Puerto Rico)、デトロイト(Detroit)で行われた。(Silver and Stowers, 1964; Detroit Metropolitan Area Traffic Study, 1955/1956)
※S was(V)  C. の構文。