Relative quietness of electric or hybrid electric vehicles is one of the most important topics in environmental acoustics recently. Reduced vehicle noise is eligible for urban sound environment, however it is also concerned as a problem because the reduced noise can lead to potentially dangerous situations for pedestrians when an oncoming vehicle is inaudible due to background noise. As a countermeasure toward the problem, regulations regarding additional warning sounds for the quiet vehicles have been developing in Japan as well as in global. Actually, Japanese government has published the guideline that recommend automobile manufacturers to equip additional sounds for quiet vehicles. This paper sorts out the root of the problem and reviews some related study concerning the additional warning sounds for quiet vehicles including the series of studies on feasible sound level of the possible warning sounds in urban environment conducted by the author. In those studies, the sound level of possible warning sounds was adjusted in some urban background sounds in a laboratory environment. The result showed that the levels clearly and reliably audible are around equivalent noise level of the background and are approximately 10 to 15 dB higher than the audibility threshold; therefore the sound emitted at an adequate level in a quieter environment could be inaudible in an environment that is 10-15 dB louder. The countermeasure using using additional sound could solve problems in limited situation, however the other countermeasure not using sound shall be considered as well as using sounds. This paper also provides a brief overview of current trends of regulation development and some researches on designing feasible warning sound, and discuss on the effect of such sounds to future urban environmental noise.