For London I started seeing more barriers in the wake of two similar vehicle attacks in 2017: https://en.wikipedia.org/wiki/2017_Westminster_attack https://en.

For London I started seeing more barriers in the wake of two similar vehicle attacks in 2017: https://en.wikipedia.org/wiki/2017_Westminster_attack https://en.wikipedia.org/wiki/2017_Westminster_attack https://en.wikipedia.org/wiki/2017_London_Bridge_attack https://en.wikipedia.org/wiki/2017_London_Bridge_attack