"You know, I find it very hard to get motivated to get on the home trainer now. I mean, aside from the occasional deluge here, there's pretty much nothing to stop me getting out on the road anytime I like".
The testament of a friend referred to the experience of cycling in Singapore.
It's a small island, covered almost completely with a sprawling metropolis, with traffic behaviour that leaves most cyclists despondent at best, and foaming at the mouth at worst. I think the official figure is something like 1.5km of road (excluding highways) for every traffic light - they don't like to delegate responsibility here!
Having said all that, there are actually a few decent stretches of road where you can find several kilometers of uninterrupted tarmac and, at certain hours, less traffic. The topography ranges from pancake flat to moderately undulating, with even one or two places where you can find brief examples of gradient steep enough to do short hill repeats on, so there is a fairly good potential for bicycle training.
There are great advantages to the flatness of the terrain. You are pedaling constantly, and your hard/easy efforts can be arranged according to your purposes, rather than being at the mercy of the gradient. With this in mind, you can easily see what my friend was referring to with the above comment.
Me, I love hills. I'm not good at them, but as Chris Boardman (I think it was him...) once pointed out: "hills are a cyclist's natural enemy". I'm a cycling Don Quixote. The other day I almost fell off my bike at an altitude of around 1800m attempting a short stretch of road that had a gradient of more than 25% with my 39x28. I failed miserably to conquer the beast but I'll be back. With more watts in the bank and armed with a 34-tooth chainring. I'll get that sucker! That gives you some idea of the mindset. I'm not alone.
Anyway, I was always of the opinion that to be good at hills you had to be dealing with them on a daily basis. Now however, I am in the unique position of having 2 distinctly different training grounds: sharing my time almost equally with one home in the east of Singapore and another in the north of Kuala Lumpur, I have mutually exclusive riding agendas of either hills or flat depending on where I am.
It's true that I get used to riding hills when I'm riding them daily - the variable cadence, rearward-seated position, out-of-saddle efforts all get plenty of practise, and this is invaluable experience. But every hill has a top, and once you get there, you don't have much choice but to go down - which means usually much less pedalling.
The cyclist who wishes to follow a structured training program, and whose training terrain is mainly hills, has much more reason to invest in a home trainer than the one who has mainly flat terrain. There's just no other way to take control and keep efforts specific. If I go out on a 3-hour ride in Singapore, I am pedalling more or less constantly for 3 hours, whereas a 3-hour ride in my Malaysian environment would inevitably include a couple of hills. This means A) it's a challenge to keep an easy ride purely easy, and B) I can't keep the effort constant.
Of course downhills also require some skill. You have to build confidence in your ability to go fast down a hill without braking every few meters. Technical descents with lots of bends require a lot of practice. It's also true that once you develop this confidence you will probably be able to put more effort into the downhills instead of just freewheeling all the way. That will also reduce some of the down time in the ride.
The bottom line: Flat terrain is better for training. You can almost fully develop yourself as a cyclist on the road on flat terrain using your gears alone to create the resistance required to develop the force and power to deal with most potential cycling challenges. The only thing lacking would be the biomechanical adaptation to the riding positions for climbing longer hills, or out-of-the-saddle efforts.
On the other hand, we are just talking about training here. Everything I've said so far would make no sense to someone who rides a bike purely for the enjoyment of it - and after all (hopefully), we all have that element in us. Ideally, we would have an environment which gives us both the inspiration to ride, and the chance to work on developing ourselves as cyclists if we so wish.
If I'm constantly flitting back and forth between my two homes as I do now, I enjoy and benefit from what both have to offer. If I spend a long period at my home in the hills, I find it hard to accurately accomplish certain types of workout that require repeats of a specific time at a specific intensity. I will have to put in some regular time on the trainer to get those done.
If I spend more than a couple of uninterrupted weeks in the flat city, I will not get the chance to work on things like the lower-cadence intensity that a long hard climb demands, but I can cover most other bases in my developing fitness. The main problem will be lack of stimulation - I get stir-crazy in such a repetitive environment and rides longer than 3 or 4 hours are a serious challenge to mental alertness. Without a regular fix of the quiet hilly roads I lose my impetus to ride.
You can see that I actually have a fairly ideal situation for a cyclist from a purely achievement orientated perspective. But ultimately, if I had to choose one, it's clear that it would be the hills that would win. Only there do I connect with my true love of riding a bike.