Affordable Long Range (15 - 30m) Distance Sensor

I am working on a project that requires frequent distance measurements at a distance of 30m (could go as low as 15m though). To give you a better understanding of what I am asking, I want to measure the distance between cars and the back of a bicycle. This requires that the sensed area is semi-wide (larger then a single laser) but not to wide (as I dont want superfluous information).

I have done a lot of research on the subject, and have been unable to find an affordable sensor that does this well. The options appear to be using a laser (which i would like to avoid as LIDAR tends to be expensive, using single lasers isnt a wide enough area, and they have higher energy demands), switching to a raspberry pi and getting measurements with a webcam (also energy intensive), or using a microwave/radar sensor. I would really love to pursue the last option.

I have an HB100 w/ effective range about 15m, but it requires lots of amplification resulting in a huge amount of noise making it more or less ineffective. If anyone has any tips for using the HB100 effectively, I would love to hear them?? If not, does anyone know of a sensor or have another approach to solving this problem???

Define affordable.

The HB100 microwave motion sensor does not measure distance to an object, but some versions or modifications allow its use to measure relative speeds between objects.

Do not put a laser out on the road regardless of vehicle. That's a bad idea.

Do look into metal detectors. A big enough loop to get 30m might make a good bit of drag.

I used to count 2 car lengths notice as good for the ones riding too far my side of the lane, mostly by ear. Maybe a parabolic/directional reflector with microphone, facing backward could detect car/road noise level?

Thanks for the reply:

I hadn't thought of either of those options but I can think of a few challenges. 1st would be ambient noise in the city/ noise from cars passing in the other lane. The second would be metal from cars parked on the road. Are there anyways to get around this?

Directional microphone. Output would be a head's up flash. If it keeps flashing steady, you might be followed.

Ultrasonic is good for 4 to 4.5 meters.

Radar... go for it if you can but it won't be easy to get working without a guide.

I knew riders who wore helmets with dentist mirrors for rear view. I used my ears and time to time turned my head.