Traffic light systems (TLSs) are used in different energy technologies for limiting the strength of induced seismicity. The TLS concept is based on real-time monitoring of induced seismicity in combination with operational mitigation measures that are triggered if the intensity of the seismicity exceeds certain threshold values. We use observations, conceptual and numerical models for investigating efficiency and limitations of TLSs. Most notably, TLS efficiency can be limited by trailing effects caused by post-injection pressure diffusion and stress concentrations at the periphery of previous seismic activity. The latter ‘stress memory’ can be the cause for seismicity occurring a long time after reservoir activities stopped. We speculate that a similar type of stress concentration could have been the nucleus of the Mw=5.5 earthquake at the Pohang geothermal site.