Why concrete barriers have curved surface profiles?

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so as to allow the vehicles that hit them to slightly go up on the barrier but not overturn. They also prevent the vehicle from again getting back on the road by rebounds. This helps in vastly reducing the chances of other vehicle hazards.

 

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The concrete safety fencing are made to contain vehicles in their carriageway, reduces the chances of rebounding into the roads, reduces to speed & leading to more hazards. Curved fencing absorb as much energy as possible henceforth reducing the impact on the vehicles. They also prevent the vehicle from again getting back on the road by rebounds. This helps in vastly reducing the chances of other vehicle hazards.

l ageee with mushtaq answer

concrete barriers have curve surface profile to reduce the sudden impact of vehicles.

concrete barriers have round profiles so cars and vehicles will bounce off it

square profiles takes the full impact and on rounded profiles the impact is distributed 

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I agree with mr. murcy answere.

To prevent massacre damage, as it take the vehicle attached with it as Eng Mursi described clearly, just to lessen the damage.