The most common and most likely answer to this crossword clue is the 5 plus 5 plus 4 letter word GREEN CROSS CODE
I've seen this crossword clue in Financial Times
G
R
E
E
N
C
R
O
S
S
C
O
D
E
More clues leading to the result GREEN CROSS CODE
- Penned by novelist, put out fake safety guide
- Naive to get vexed over third rate poem as an aid to road safety
- Growing frustrated with cipher? This should prevent people in the way getting hit
- Set of safety rules, new, annoyed convention
- Set of safety rules old codger screens once rewritten
- Fresh vote — a signal for proper pedestrian safety
Buddy explains!
The straight part
We know that "an aid to road safety" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an aid to road safety = green cross code.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.