Is a Purely Rational World a Technologically Advanced World?

If we were purely rational, then we could be trusted a lot more with dangerous technologies. The tough case is when anyone smart enough to invent something big is made rational, and ordinary/dumb people are as they are. So let's suppose just inventors are rational.

Motor vehicles are not that dangerous, compared to their benefits.

http://www.infoplease.com/ipa/A0922292.html

The peak death rate by motor vehicle peaked at 269 per million in 1970. The motorization of ambulances alone saves far more than one in a thousand.

Further helping, we'd have had seat belts and air bags and crumple zones, forward radar auto-brake, automatic lane centering, etc. much sooner. And, for that matter, we'd have been working on efficiency and alternate energy sources, thereby mitigating the other drawbacks.

We probably wouldn't have dismantled our light rail system, too.

Would pure rationality have severely limited the advancement of technology?

I doubt it. Technology is a tool, and turning down tools doesn't sound that rational as rules of thumb go. If there's some strict advantage to not using technology a lot of the time (like playing to our evolutionary preferences or something), you should still have it around even if you keep it locked in a vault. Still, you'll need it eventually to stave off some mortal sickness or asteroid or to avoid the volcano that's set to explode in six months, etc. I imagine our hypothetical purely rational agent would be far better than us at managing dangerous tech.

Strategies would be different for an individual as opposed to societies. Both would as a first approximation only be as cautious as they need to be in order to preserve themselves. That's where the difference between local and global disasters comes into play.

A disaster that can kill an individual won't usually kill a society. The road to progress for society has been paved by countless individual failures, some of which took a heavy toll, but in the end they never destroyed everything. It may be a gamble for an individual to take a risk that could destroy it, and risk-averse people will avoid it. But for society as a whole, non-risk averse individuals will sometime strike the motherlode, especially as the risk to society (the loss of one or a few individuals out of the adventurous group at a time) is negligible enough. Such individuals could therefore conceivably be an asset. They'd explore venues past certain local optima for instance. This would also benefit those few individuals who'd be incredibly successful from time to time, even if most people like them are destined to remain in the shadow.

Of course nowadays even one person could fail hard enough to take everything along with it. That mat be why you get that impression that rational people are perhaps too cautious, and could hamper progress. The rules for the game have changed, you can't just be careless anymore.

Would pure rationality have severely limited the advancement of technology?

It depends on what the utility function of said pure rationality was.

(I've seen the argument that average utilitarians would have never invented agriculture, as the average human in 19th century was worse off than the average human in palaeolithic times.)

What does it mean or something to be purely rational? Having a narrow notion of what one means by that will probably impact th question a lot. I'm not aware of any good definition for a purely rational agent of limited intelligence.

A purely rational person would be nigh omniscient. If a combustible engine does more good than bad (which it does), a purely ration person would realize this.

If you want to know how we'd act if we just weren't biased about risks, but were just as imprecise, consider: would it be worth while to have been substantially more cautious? Barring nuclear weapons, I doubt it. The lives lost due to technological advancements have been dwarfed by the lives saved. A well-calibrated agent would realize this, and proceed with a lesser level of caution.

There are areas where we're far too cautious, such as medicine. Drugs aren't released until the probability of killing someone is vastly below the probability of saving someone. Human testing is avoided until it's reasonably safe, rather than risking a few lives to get a potentially life-saving drug out years earlier.

Well, one has to distinguish between purely rational being that has full set of possible propositions with correct probabilities assigned to them, and a bounded agent which has a partial set of possible propositions, generated gradually by exploring starting from some of the most probable propositions; the latter can't even do Bayesian statistics properly due to the complex non-linear feedback via proposition generation process, and is not omniscient enough to foresee as well as your argument requires.