What is the disadvantage of using bollards and hedgehog barriers?

The correct choice reflects a significant consideration in the use of physical barriers like bollards and hedgehog barriers. While these structures are designed primarily to impede vehicular access and enhance security, they may not be effective against certain types of attacks, particularly those involving explosive ordnance. This vulnerability arises because they are primarily built to stop or slow down vehicles rather than absorb or deflect explosive blasts. As such, while they serve as a deterrent and can prevent unauthorized vehicle access, they may not adequately protect critical infrastructure or people in scenarios where explosives are involved.

In contrast, factors such as the cost of installation, space requirements, and the potential for these barriers to be bypassed do play a role in their overall effectiveness and usability, but they do not directly pertain to the specific limitations related to defensive measures against explosive threats. The main issue is the inadequacy of physical barriers to withstand explosive impact, which is a crucial aspect to consider in integrated defense planning.