My 13 year old daughter dives 4 nights a week at ASU. We almost always drive home by going north on I-17. Tues. night 09/18/2012 around 7:30pm we both noticed a bright blue light that was slowly pulsating on the west side of I-17. It seemed stationary and about the size of the streetlight. I think we were near the Peoria exit when we first saw it. I wanted to try to find out where this light was coming from but my daughter had too much homework that night. I estimated that it was between the Thunderbird and the Greenway exits but I couldn't say it's altitude or how far west of me it was. I watched it until it was out of sight in my mirror. The next night we looked for the light and on that night it was just after 8pm when we passed the area but there was no blue light. I just want to know if anyone can say what this light is. I have not seen the light prior to Tues. and we have been driving that way for 20 months, nonetheless I would think there is probably a logical explanation especially since it was hovering there for many minutes.