The most common and most likely answer to this crossword clue is the 8 letter word HIGHWAYS
I've seen this crossword clue in The Daily Telegraph
H
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More clues leading to the result HIGHWAYS
Buddy explains!
The straight part
We know that "where driving is fastest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where driving is fastest = highways.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does HIGHWAYS mean?
HIGHWAYS noun- A motor vehicle for transporting large numbers of people along roads.
- An electrical conductor or interface serving as a common connection for two or more circuits or components.
- (medical industry) An ambulance.
- A main, direct public road, especially a multi-lane, high speed thoroughfare connecting major population centers.
- Any public road for vehicular traffic.