Hi Eileen, I'm afraid I have no idea about the Turrell question. I haven't been able to find any reference in the Centre Civic Convent Sant Agusti website either. I however found an external reference of this exhibition in another website, http://barcelonafreeart.net/2011/01/15/war-and-art-piece/ but it's dated in 2011 so I am afraid you're far too late for this (unless there's been a rerun or something!). This art centre renews its exhibitions very often so most of them are in display for a (relatively) short period of time. Same then in Palau de la Virreina or Centre d'Arts Santa Mònica, both in La Rambla (same style, mostly free exhibitions).
It's indeed interesting the story about Oahu, especially taking into account it throws us into a complete different set of problems here: sustainability. In Europe this is an issue very much into everybody's agenda, not just because we all must do our bid to 'safe the planet' but also for an equally important yet more practical matter: traffic. In many of our major cities traffic is near collapse and needs to be reduced drastically. Also, some of these old cities, having exploded in size and population in the late 18th and early 19th centuries are not 'designed' to cope with lots of traffic due to lack of space. Thus, encouraging non-residents (tourists) to renting cars would be a reeeeeeally bad idea for us. Instead, a push on enhancing public transportation systems is more or less established everywhere... with some places being more successful than others I must say. We also have in Barcelona a few car share 'experiments' (ie. Car&Go, not "two" but "and" ;) but it doesn't seem to catch up (yet!). In Barcelona, probably also due to its weather, one of the more recent successes is the public bicycle sharing scheme set up by the City Hall several years ago: https://www.bicing.cat/ca/content/qu%C3%A8-%C3%A9s-bicing Unfortunately for you, it's aimed to the city residents only. Having said that, there is a blooming private business around the bicycle rentals for visitors too, yet at another sort of prices of course as the public scheme is, again, partly subsidized... of course :)
Barcelona is a relatively compact city -in fact it has one of the biggest densities among all the major cities in Europe, the city has 1.7 million inhabitants in 40 sq miles, no less than a little over 41 thousand inhabitants per sq mile -only surpassed by Paris and Athens, albeit the population of the later is less than half the size of Barcelona's. (http://en.wikipedia.org/wiki/List_of_European_Union_cities_proper_by_population_density). And this of course without counting the passers-by, that is, all the people that commutes daily into the city from the metropolitan area -which comprises 36 cities, summing up to a total of 4.5 million people in an area a little over 300 sq miles. It might not sound a lot, especially if compared to major metropolis in the rest of the world, but I can tell you it's far enough for many of us, LOL!
But again, Barcelona is made for walking as everything is relatively close. There's a saying among locals that "...everything is at a distance of 15-20 minutes", be on foot, be by bus/metro/tramway, be by bicycle, be by motorcycle (yes, Barcelona is 'the' city of the scooters and motorcycles, even more than Rome itself!)
Your last thought it's indeed an interesting one but, having lived myself in several big cities around Europe, I suspect there's not a single answer as it often involves many factors other than the mere straight forward economic ones (topology and orography of the city, its size, the weather and also the idiosyncrasy of its citizens).
