The most common and most likely answer to this crossword clue is the 8 letter word MOTORWAY
I've seen this crossword clue in The Daily Telegraph
M
O
T
O
R
W
A
Y
Buddy explains!

The straight part
We know that "high-speed road" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case high-speed road = motorway.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 MOTORWAY mean?
MOTORWAY noun- (parts of Australia) A broad highway designed for high speed traffic, having restrictions on the vehicle types permitted and merging lanes instead of cross traffic; in parts of the United States and other places called freeway.