( ( The ( air quality ) ) ( ( ( became ( so dangerous ) ) ( that ( ( at ( one point ) ) ( , ( ( ( the number ) ( of ( cars ( entering ( the city ) ) ) ) ) ( was restricted ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN air) (NN quality)) (VP (VBD became) (ADJP (RB so) (JJ dangerous)) (SBAR (IN that) (S (PP (IN at) (NP (CD one) (NN point))) (, ,) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NNS cars)) (VP (VBG entering) (NP (DT the) (NN city)))))) (VP (VBD was) (VP (VBN restricted)))))) (. .)))
The air quality became so dangerous that at one point, the number of cars entering the city was restricted.
(ROOT (S (S (VP (VBG Limiting) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NNS cars)) (PP (IN on) (NP (DT the) (NNS roads)))))))) (VP (MD would) (ADVP (RBS most) (RB likely)) (VP (VB improve) (NP (DT the) (NN air) (NN quality)) (PP (IN in) (NP (JJ large) (NNS texas) (NNS cities))))) (. .)))