Free bus travel for the disabled and the over-60s was an admirable policy principle when the Concessionary Travel Scheme was introduced in 2006 and remains so today.

Yet if politics is the art of the possible, as Bismarck said, then a little bit of realpolitik would not go amiss when it comes to safeguarding the long-term sustainability of the scheme.

Bus operators in Scotland claim they have been warned they could face a shortfall of up to £15 million by the end of this financial year in the amount of money they are given by the Government to reimburse them for the cost of running the scheme. The result could be that less well-used services, especially during off peak hours, could face further cuts.

Bus companies have already cut services due to rising fuel prices and the cost of running the scheme. In 2012, fares went up while buses completed fewer miles.

It is true that operators have not covered themselves in glory since the scheme began. In April, The Herald revealed that the Scottish Government had investigated 208 cases of "overstaging" the previous year, where passengers entitled to free travel had been issued with tickets for longer journeys than they made, allowing firms to claim a higher rate of reimbursement (though the roll-out of anti-fraud measures has made this more difficult).

However, there is no arguing with the basic arithmetic. Audit Scotland has warned that due to Scotland's ageing population the scheme could surge in cost to £537m by 2025. Without an increase in Government subsidy, something must give.

The Scottish Government thus far has proved reluctant to amend the scheme, but there are reasonable grounds for doing so. It is undoubtedly true that it greatly benefits many pensioners. However, it is a confused sort of logic that deems it fair for a young person struggling in a low-paid job to pay the full fare while a comfortably off retired person pays nothing. One possible solution might be the introduction of a small flat fare for the over-60s. Pensioners would continue to benefit from subsidised, low-cost travel, but the measure would raise money to help keep less commercially sustainable routes alive, for the benefit of the whole community.

Unless subsidies are increased, or the scheme somehow amended, the unfortunate consequence of this well-meant policy is likely to be further erosion of Scotland's bus services.