We use commas to separate particular ideas? What does that mean?

[Rich7]

Test No. incompl/advan-82 "GMAT Verbal Quiz (5)", question 1

Thanks to the crash barrier in the middle of the motorway cars are ......... from hitting those on the other side in the event of an accident.

(a) distracted
(b) diverted
(c) directed
(d) prevented

Test No. incompl/advan-82 "GMAT Verbal Quiz (5)", answer 1

Thanks to the crash barrier in the middle of the motorway cars are prevented from hitting those on the other side in the event of an accident.

Correct answer: (d) prevented

Your answer was: correct
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shouldn't a comma be needed here after motorway?

Because I had a hard time understanding the hole sentence.

[Torsten]

The sentence makes perfect sense without a comma. We use commas only to separate particular ideas of a sentence. Also, please take a look at this post:
http://www.english-test.net/forum/ftopic3391.html
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