My personal best is 19.4mph over 10.4 miles. This is on a rigid mtb with 1.5 inch city slicks, riding 5 miles into nottingham city centre and through the other side in morning traffic at around 8.15-8.30 am, so lots of slowing for traffic and lights.
I like to pace myself using other cyclist, so i’ll aim to overtake them by a certain point, i find this helps me ride a lot quicker.
Added a lot of speed the last couple of weeks by dropping down a couple gears and standing on the pedals to power up the hills, this keeps my speed more or less constant.