guardrail

Short definition from Wiktionary

Noun

1. 1
   A rail set alongside a dangerous place as a barrier, to improve safety.
2. 2
   A rail set alongside a dangerous place as a barrier, to improve safety: A strengthened barrier installed on the sides of a (usually major) road to keep vehicles on the highway and reduce the risk and/or impact of crashes.
3. 3
   Any barrier designed to prevent accidents or mistakes.
4. 4
   Synonym of check rail.