A tunable 2D rod type Si Photonic Crystal cavity based impact sensing configuration is proposed and numerically analyzed. The cavity is sandwiched by perfect electric conductor (PEC) boundaries in order to provide out-of-plane light confinement. An on-purpose air slot is introduced between the Si rods and top PEC plate moving the light confinement into the slotted region and making the cavity highly responsive to the displacement of top PEC boundary. Optomechanical coupling strength is calculated to be on the order of 300 GHz/nm. Proposed light confinement inside the slot shows similar characteristics to slot waveguiding phenomenon and offers valuable opportunities for mechanical sensing applications. For a practical approach, PEC boundaries are replaced by Gold plates and the potential of the structure as an inertial sensor is investigated with a specific focus on impact sensing applications to be used in automotive security systems. Numerical analyses indicate that the device, whose sensing area is only 106.6 mu m2, has a response time of 16.6 mu s asserting that the proposed sensor can sense the presence of an impact faster than several commercially available ones, in a much more compact form.