1. https://en.wikipedia.org/wiki/File:Concrete-step-barrier-M18.jpg.
2. http://i01.i.aliimg.com/img/pb/933/001/296/296001933_554.jpg.
3. http://media.mlive.com/kzgazette_impact/photo/9260974-large.jpg.
4. http://www.mainlinefence.com/gallery/category/guard-rails.
5. Committee on Guardrails and Guide Posts. Proposed full-scale testing procedures for guardrails. Highway research correlation service circular 482. Washington (DC): Highway Research Board; 1962.