[jnty]

But it would only decrease traffic and pollution for the duration of the spend. Once it's over, those benefits stop and there is nothing left beyond the lingering effects of the intervention.

It's actually worse than that, because you've essentially taken out an unsecured loan of £100bn and spent it on something which neither increases ongoing tax revenue or asset value, so you now have to cut other spending to pay it off. Which is why governments and businesses separate capital expenditure (building things) from current expenditure (doing things) very carefully.

[onion2k]

But it would only decrease traffic and pollution for the duration of the spend. Once it's over, those benefits stop and there is nothing left beyond the lingering effects of the intervention.

The lingering impact of millions of people realising that bus travel is actually a good idea and that cars aren't necessary in a lot of cases. How terrible!

[jnty]

You're misunderstanding my point. Transport subsidy funded from current income is a good idea. Spending your entire capital infrastructure budget on making it free for a few years is not. The 'lingering benefits' don't linger long if you then have to hike fares massively because you've got a big loan to repay as a result with no new assets you can exploit to service it.

It's for this reason that saying 'we could spend the money on making buses free for a N years' is meaningless. It would be more sensible to say 'we should spend the £Nbn we spend on maintaining the motorway network on free buses' because those types of spending are equivalent.