I do a lot of my long training rides on a long straight out and back route with a few hills but no real climbs. Turns and traffic kill speed so the less of that the better

I don't think a velodrome is ideal. It can get tiring taking the corners at speed in the aero position.

I think on centuries some hills are a good thing. Sure you have to work harder on the way up, but you can always coast the downhills to get a short break.

My fastest centuries were actually both halves of a double century with about 5K of climbing over the 200 miles. Not too hilly, but not totally flat. It also helped that I found some nice groups to ride with. Being able to sit in a group and draft for large parts of 15-20 miles at a faster pace than I would go solo helps boost your average speed.

I would use a paved rails-to-trails type road, if you have one. In Georgia, there is a nearly flat rails-to-trails that is about 60 miles to the neighboring state. Once those trails get rural, there are few intersections to stop you. I have done an out-and-back century on this

Fastest ride will be flat with no stop signs/lights and tail wind both ways. Since that likely won't happen, if you have predominant winds in your area, use them to your advantage. For me, I live where there are predominant winds coming out of the south to southeast. The winds are calm/light in the morning and pick up steam through the day. It's not uncommon to go out with calm weather only to have 10-15 MPH tail winds 3 hours later. Fortunately all the good riding roads are south. To the north are millions of people, tons of lights, etc. If wind speed and direction don't change much, the fastest will be flat and calm. Out and back with elevation change or tail/head winds will both be slower compared to flat and calm for the same power.

Big thing is having extra bottles with you to avoid stopping.

In my first year of riding I wanted to break 5 hours for a solo 100. Went to a local park that has a 1.3 mile circuit protected by wind and offered shade. Also gave me quick access to supplies as needed with my car no more than 90 seconds away. It was boring as hell doing that loop for nearly 5 hours, but I did it.

If I was going to try that, there is a park in my area with a flat 5 mile loop with no stops and only one spot where you even have to think about traffic and even at that spot I have never had to stop. I've done 45-50 miles there several times and I once did a 125 mile ride there but that was an event with tons of people. That length of loop is long enough for me that it does not get any more boring than riding on any flat straight road. Being able to pass your car on a regular basis would make resupply much quicker and save you several minutes even if you minimize your stops. Even a "quick" gas station stop is going to take you 5 minutes. Plus, it is pretty hard to plan a route that is both fast and has a stop right where you want it.

Bingo, that is exactly what I did last October on a whim. I rode from the start of the Silver Comet out to Cedartown and back. I was flying on the way out, but the wind and some fatigue got me on the way back.

I plan to do the ride to Alabama and back, whenever time allows this year or next year.

Again, a dedicated path/trail (paved) would seem to be ideal. If I am not mistaken, Richmond, Virginia has one of these types of trails that is exactly 50 miles long. Obviously, out & back would get you a century.

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Pink? Maybe. Maybe not. You decide.

If you live near water/coast, sometimes you might find the wind shifts from morning to afternoon.
There is an out/back century route in coastal MA/NH where sometimes you can get tail-tail winds; cycling nirvana.
Of course if you hit it wrong and get head-head, it can be cycling hell.

Yeah, we have a few possibilities like that around here, but they are multi-use (pedestrians) so I generally avoid them. But on a low-traffic day they may actually be pretty good.

I guess the most obvious solution is to increase fitness so 20mph average isn't 90+% effort.

I do a weekly 50mi, generally rolling hills along the coast and one summer fought my hardest to break a 20mph average, never did it. I put in more miles and hills through the winter/spring, the next summer I was fighting to break 21mph, again never made it. The second summer a 20.5 average felt like an off day where I was physically and mentally tired, even though the previous summer I would have killed for that 'off' time. All these rides were on an aero road bike, 30mm aluminum wheels.

The ride: 49.5 mi, 1600 feet, mix of bike path and coastal road - minimal traffic lights. Obviously my fastest times are low wind days and no traffic lights. I can do 50 mi on 2 bottles and one bar so that really simplifies things, but I'm trashed at the end of that ride.


Great, now you have me thinking about trying to break 20 on a solo 100. Probably will have to use the TT bike...

Where do you live? If you are in Colorado (or willing to travel there) try the Elephant Rock Century in early June. It's fairly flat, the roads are closed and there are several aid stations along the route.

It may not work for a solo shot, but I would add that the Cap2Cap in Virginia is pretty flat and has a group that is doing this in the 4:15 range.

Do the "Sub 5 Century". You'll have to google it, I don't know how to attach a link.

It's in IL. Buddy of mine did it a few years ago.

If you're not limited to the US then I'd highly recommend https://www.visitdubai.com/...-qudra-cycling-track :)
Seriously it's over 100km of of almost flat perfect smooth tarmac bike path in the desert. With enough options you could even choose how to play the wind whilst never seeing a car or a stop sign!

In the US I'd suggest the Yosemite valley loop starting early on a quiet day in late autumn when there are no tourists. Great tarmac, nearly flat, there is a loop with no stop signs and at altitude.

That looks amazing (and fast).