Ask HN: After-market forward collision warning systems for older cars?

Does anyone have positive/negative experiences adding after-market forward collision warning systems to their older car?

I have a 2015 MPV and just had a minor scrape on a lane safety barrier because I got distracted and drifted for a split second (I know - bad driving - I learned my lesson - I'm normally a fine driver but was having an off day). Anyway, so I was thinking, it should be fairly trivial for a camera system to detect the imminent impact and issue a beep to alert me to my slip-up, but I can't seem to find any online. Mobile-eye no longer seems to sell them. Does anyone have any experiences to share, why this is or isn't a good idea?