In this paper, based on the vibration energy harvesting technology, we introduce a battery-free pavement roughness estimation system (BF-PRES), which provides road roughness information by linking driving vibration and wireless packet count. Instead of harvesting energy to power an off-the-shelf commercial accelerometer and implementing some algorithm for vibration estimation, BF-PRES simply sends out a BLE Beacon packet, when the accumulated energy arrives at a sufficient level. Given that larger vibration intensity gives higher harvested power, the packet count within a constant time interval should be positively related to the road roughness. Lab testing shows the feasibility of the proposed design. This study provides a new design scheme and easy implementation of battery-free or energy-constrained IoT systems.