VS,
thank you for your reply. I have only just now had time to look into this properly. Thank you also for the helpful graph. I did not realise that the speed bonus was interpolated: I thought that the bonus specified applied in from the year specified until the new bonus was specified. I have not immediately been able to find where my file goes off the scale, though - if it does, it certainly needs adjusting.
I think that we have slightly different aims in our balancing, however. I specifically designed my speedbonus.tab file to work with my speedbonus patch,
here. The result of that is, because local transport is taken out of the speed bonus system entirely, the speed bonus is set more aggressively, because, instead of having to take into account
all vehicles, it now only has to take into account vehicles designed for long-distance travel (with the patch applied). I have been testing it: with your speedbonus.tab and my patch, there is
more of a gap between local and long-distance revenues than there is with your speedbonus.tab and the default binary (local revenues being lower with my patch applied). With my speedbonus.tab and my patch, however, revenues for local transport are higher (with the local bonus set to 25%) than they are with my speedbonus.tab and the default binary, which was the intention of the patch in the first place. So it might well be that your speedbonus.tab is more appropriate for the default version, and that my speedbonus.tab (probably with some adjustment, in light of what you have told me about interpolation) is better for my patched version.
Incidentally, I think that I need to re-think the approach to aircraft. In the speedbonus.tab that I uploaded above, I treated aircraft just the same as all other forms of transport, and set the bonus quite aggressively in comparison with the fastest available in the fleet. However, it strikes me that, because of the nature of air travel, aircraft should
always attract a very large speed bonus: in reality, people pay far more per kilometre to travel by air than any other form of transport precisely because air travel is so much faster; aircraft also cost a great deal more to run than land or sea transport. Therefore, air transport should have very high running costs, but should have its speedbonus set to very lax values (hundreds of kph lower than the fastest currently available, and with, perhaps, no significant increases after the 1960s/1970s), which should give a realistic and workable balance.