Commute fuel economy.
 

Just as a matter of interest, I gathered the ScanGauge data after running my wife to work this morning. I averaged 61.8 mpg(UK) for the 3.9 mile round trip. This was from a cold start and the water temperature never quite got up to normal operating temperature. My average speed was 20 mph, a maximum speed of 33 mph and the maximum revs were 2512 rpm.
I have to reiterate that this was at 06:30, and despite all the miles being in town, with roundabouts and traffic lights, there was very little traffic. Conditions were dry and cool.
This shows the benefit of a route that has been well learned and where I make use of Engine On Coasting and DFCO.
Only times I stopped were to drop my wife off and at one set of lights (switched engine off in both cases). This shows the benefits of roundabouts here in the UK (15 in the 3.9 miles).

Maybe it's due to their being so few of them or it's that people are so rude and self-centered while driving but roundabouts make me very uncomfortable. I'd rather stick with standard roads even if it does take a hit on mpg.

They put one in here a while back, but it is too small. You don't need to turn much, and thus can zip through without slowing down. Which people do.

We have proper roundabouts but also lots on mini roundabouts. They are just painted humps (sometimes not even humped) and they are sometimes abused. Lots of moronic drivers don't even make a show of going round them. They do have all the legal import of a "real" roundabout so if you are involved in a bump at one, the blame can be assigned.

Roundabouts make me nervous. I'm never quite sure what it is I am supposed to do. If they were common, I'm sure it would be no problem, but it's a real rarity here in the US, and I really believe that rarity makes them a hazard.

In the UK, when entering a roundabout, it is just a case of giving way to vehicles approaching from the right(UK), and vehicles already on the roundabout. I do agree though, if roundabouts are few and far between then motorists could be unsure of the rule.

Another think that makes a big difference to safe working of roundabouts is correct signalling. If drivers follow the rules on signalling as published in "The Highway Code", then traffic flows really well. Unfortunately, there are many kn*bhead drivers, here in the UK, who are too selfish to signal, leading to other motorists being held up while they see which way they are going.
Speed is also an issue. If drivers treat roundabouts like the junctions they are, then fine. Unfortunately, many treat them like a speedway.

Agree 100% with you there, a good 85% of people don't signal at all. I've had a few near misses where I've assumed the person is going straight on and exiting, when they're actually carrying on. I'm always sure to blast with them with my horn. The small roundabouts here aren't an issue, but when I go further afield, the larger multiple lane multiple exit ones make me a bit nervous, I'm always worried about being in the wrong lane and cutting people up.

Something to note is how much safer roundabouts are, as all approaching parties are forced to slow and give way, and you only have to give way in one direction. There was a campaign a few years ago, two junctions locally where for some reason people found them really confusing and kept pulling out into oncoming traffic and killing each other. We had two roundabouts put in and there hasn't been a single fatality as of yet.

There are some corkers of roundabouts up and down the country.
This is one I use every Saturday, once in each direction.