"The hood of most vehicles is usually fabricated from sheet metal, which is a compliant energy absorbing structure and thus poses a comparatively small threat. Most serious head injuries occur when there is insufficient clearance between the hood and the stiff underlying engine components. A gap of approximately 10 cm is usually enough to allow the pedestrian’s head to have a controlled deceleration and a significantly reduced risk of death[3]. Creating room under the hood is not always easy because usually there are other design constraints, such as aerodynamics and styling. In some regions of the hood it can be impossible. These include along the edges on which the hood is mounted and the cowl, where the hood meets the windshield. Engineers have attempted to overcome this problem by using deformable mounts, and by developing more ambitious solutions such as airbags that are activated during the crash and cover the stiff regions of the hood[4]. Some models, like the Citroën C6 and Jaguar XK feature a novel pop-up bonnet design, which adds 6.5 cm (2.5", C6) extra clearance over the engine block if the bumper senses a hit. In 2012 and 2015, the Volvo V40 and the Land Rover Discovery Sport have a under-the hood airbag with operates if the hood senses a hit. The airbag also covers the windshield pillars to provide better protection in all areas struck by the pedestrian's head." there's a great wikipedia article about it.[1]
[1]https://en.wikipedia.org/wiki/Pedestrian_safety_through_vehi...
A car like the mid-70's Aston Martin Lagonda with it's knife-edge grill will never be built again.
https://upload.wikimedia.org/wikipedia/commons/d/d6/Aston_Ma...