Q: What do normal people drive in the Imperium?
A: Nothing, unless it's their job to drive it.
I'd say with a high degree of conviction that civilian ownership of ground cars on a planetary scale is not a thing in the Imperium. Ground Cars are common, I have no doubt that they exist on most Imperial worlds and could be hired by the PC's cheaply, but the average Imperial Citizen would simply not have the means, skills, or even the right to own one. Anyway, I'll list my assumptions and move on.
- The average developed Imperial world seems to have extensive, affordable (or even free) public transport systems. This makes sense to me because the Administratum is concerned primarily with the Imperial Tithe on any given world, so productivity is not only important for an Imperial Governor, but vital to their long term success and survival. Details like safety and quality obviously vary from world to world, but getting between places where people have to work or sleep probably isn’t hard.
- Centralized public transport is easier for an organization like the Adeptus Mechanicus to maintain and supervise than a multitude of privately owned vehicles. The AdMech wouldn't care about running the grav-rail or whatever it is themselves, but they sure as hell wouldn't teach you how it works or how to fix it, that would be Tech-Heresy!
- For most citizens, knowing how to repair a ground car is probably Tech-Heresy.
- The average citizen won't have a great deal of time for recreation and very few rights or civil liberties, if any. In the real world, common reasons for an individual to have a car are for things like work needs, long distance travel, and because they provide you with the personal freedom to go where you want in a reasonable timeframe. In the Imperium, your work needs are met, and you probably won't even have the right to go out and experience the world to begin with.
- Ground Cars are expensive, Imperial citizens are poor and downtrodden. You may be trained and expected to operate a groundcar for a corporation, adepta or other master, but you're unlikely to ever be able to afford one of your own.
- Almost none of these rules apply to the Nobility, if you encounter a privately owned speeder, odds are it's owned by a Noble.
See, I disagree with a lot of this.
-In the Ravenor novels there are sequences where the characters fight in and around a major road, with what appear to be thousands of vehicles passing along it.
-In the same series of books a relatively lowly law enforcement officer has an elderly relative who is at best middle class who owns a powerful old groundcar.
-There are many references in the Enforcer series to extensive road networks.
-Whenever Gaunts Ghosts stop fighting on a halfway civilised Imperial world, they end up using staff cars, and in Blood Pact steal an old truck owned by a backwater undertaker.
I agree entirely that on most Imperial worlds, groundcar use is going to be very different to vehicle use enjoyed by modern westerners. But I think it's still relatively widespread on worlds that have the tech base and environment to support them. I'd exclude feral worlds from this, and a lot of hive worlds (either because space is limited in the habitable regions, or because the environment precludes their use) but on mid-tech and upwards frontier worlds, many hive worlds and many Imperial worlds, groundcar use would, I'd argue, be a fact of life for many citizens.
One concept that makes me think groundcars might be relatively common is the concept of "archprints." These are blueprints that the Adeptus Mechanicus effectively "franchises out" to local tech-guild on Imperial worlds in return for an enormous fee. These tech-guilds can then build and maintain licensed technology, within strict rules as to what they are allowed to manufacture. This concept appears in many Abnett novels, and actually hangs together as a concept beautifully. One can argue that the economy of hives like Gunmetal is likely based upon such licensed archprints.
OK, so up to now, I've been citing canon, but now let's extrapolate further, and take things off-canon for a moment.
-we know the Adeptus Mechanicus produces "archprints" and sells them to local tech-guilds
-let's say that the Adeptus Mechanicus produce archprints for a series of cheap and low-powered groundcar engines, and permits licence holders to coach build their own chassis.
-This would instantly create a huge potential variety of cars - but all of them would be powered by the same engine. Tampering with the holy form of that engine would be tech heresy… but maintenance and repair of that engine would be something that the local tech-guilds would be permitted to do by the Adeptus Mechanicus.
-So, depending upon the types of engine locals are permitted to make, you could end up with all kinds of weird and characterful vehicles. Like motorbikes powered by tractor engines. Or buses powered by bike engines. Or tractors converted into armoured vehicles. (Oh hang on - that does appear in the canon.)
-This means you have STC technology forming the basis of a variety of characterful and unpredictable local vehicles.
I know I've wandered off canon there…but i believe within plausible limits. Plus, rule of cool applies. Why restrict your players' access to car chases? Car chases are cool.
Okay, before I drop the boring block of text; I love the rule of cool, and car chases are great. We've had car chases in our games of Dark Heresy and Rogue Trader and they were pretty frickin' sweet. Car chases are awesome, so have car chases, I say!
The sole point of my longwinded argument is that while groundcars are common, mainstream ownership of them by the civilian population seems highly incongruous to me. It isn't directly confirmed, denied or even commented on very often, even on a planet to planet basis. Because of that and the constant implications and references to abject poverty, I don't believe that Joe Citizen is likely to have one. Your points are all fine, reasonable and very plausible to me, not to mention the fact that the Imperium is a hugely diverse empire that really doesn’t give a crap about most of the things that happen on any of its worlds, provided their handful of well documented requirements are met.
Most developed worlds, according to the bulk of sources I'm familiar with (which doesn’t include the Enforcer series, incidentally), seem to support the loose rule that the average Imperial citizen is little better than an oppressed peon who may or may not even be literate. The entire planet he or she lives on is likely to be a dystopian craphole run by fascists, fascists who are contextually the good guys and serve the great and righteous space fascists who rule from Holy Terra, no less.
Most developed worlds are hive worlds as a direct result of Imperial control, or on their way to becoming hives, which are horrendously mismanaged, over-centralized and generally pretty horrible to live in for the majority of the populace (the "Accustomed to Crowds" rule hivers have in Dark Heresy springs to mind). I’m not saying that there can’t be or is not a large percentage of the million-and-more worlds of the Imperium where private ownership of a groundcar by an individual worker or family unit is at least theoretically possible, just that if its allowed, it's probably fairly expensive and may not even be a boon to them on account of the various inefficiencies in the system and general neglect of the authorities. And yet there are groundcars all over the place, even on worlds that clearly establish that the citizens are poor, uneducated, downtrodden illiterates who couldn't ever afford one. I have no idea who needs so many of ‘em, sometimes they’re just there even if that makes no damn sense.
But honestly, this is 40K; there are space chimps that make super compact weapons, laser guns that shoot bolts that travel at slower than the speed of light, nuclear weapons are considered almost comical as a means of attempting mass destruction, there's ludicrously over-engineered tech left, right and centre administered by people who pray to their toasters, spaceships that fly through hell to get to their destination quicker and *evil is a thing you can catch like some kind of disease*. I’m probably not even listing any of the really crazy stuff.
Just remember that in general, while the Imperium is hugely diverse and most developed worlds will be choking in groundcars, the basic rule for the common Imperial citizen is that love for the Emperor is mandatory, your "betters" exploit the hell out of you, life sucks and you’re damn lucky it doesn’t suck even more!
I would honestly bet on the Acolytes being able to legitimately acquire and use a groundcar on any of the core worlds at the very least, but if I had to name one Calixis Sector planet where pretty much anyone could own and keep a vehicle, I'd bet on it being Iocanthos; it's basically Mad Max planet, so pretty much anyone who had to leave Port Suffering would have to have rugged all-terrain cars to get places (not to mention be in theme!).