If you have a GoPro, use that!

I've used the footage from my GoPro to get very decent lap time feedback. Not at all sophisticated, but works like a charm - just load the video in a program that lets you go frame-by-frame such as Avidemux, calculate the time between the same reference spot (such as the starting line, and you'll be within a tenth of a second or so easily. Usually when I am bored on a Monday following races, I'll go through my footage and my lap times calculated from the video is consistently within 0.05s of what is eventually posted online. Plus nothing beats a Monday like reliving the races frame-by-frame!

Jamie