Road pedals

I used Shimano SPDs from when they first arrived in the UK (about 1990), always had slightly iffy knees but had to make the switch over to Time ATACs about 11 years ago. When I finally got a road bike I naturally thought Time pedals (on advice from shop - who built my bike & who I trusted, I did spec Time Rxs).

From my experience, as the owner of dodgy knees, who had to go Time ATAC, Time RXS pedals are very similar to ATACs. I've had no problems

Agree with all of the above about position - i have rubbish knees, and changes in position between different bikes can often aggravate things for me. If there is a good bike fitter near you, it's well worth the money to avoid ongoing problems.

I also use Speedplay because they are kind to my knees - the big difference for me is not just the amount of float, but how free it is. There is no spring tension at any angle within the float on my Speedplay Zeros, meaning my knees are free to move as they please without being twisted by the pedal. My knees love them.

before you bin your pedals, you need to check your position on the bike. the angle of the cleats, the fore/aft position on the foot, the length of the crank and the fore/aft position and height of your saddle could all affect your knees if they're not right. if you just replicate your setup with a road pedal, chances are you'll get the same result.

Most of the road systems are pretty similar to be honest so it comes down to personal preference on looks and (more importantly) the price of replacement cleats. A larger surface area between the cleat and pedal body gives good support when riding compared to SPD's but make them rubbish for walking in. Most systems have around 5-10 degrees of float but a lot of people with knee problems (myself included) use Speedplay as they have 15 degrees of float. Its worth checking your position first though to make sure nothing is amiss there putting strain on your knee.