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.