The Scottish Government will miss a key target to remove the majority of diesel buses from public transport by next year.
Transform Scotland found only about 16% of the bus fleet will be electric or hydrogen-powered by the end of 2023.
Its analysis of 10 transport goals stated that, while some of the commitments were on track, progress in other areas had slowed or stalled.
Transport Scotland said it was "right to aim high".
The BBC says the SNP's 2021 Holyrood election manifesto pledged to "remove the majority of fossil fuel buses from transport in Scotland by 2023".
The ambition was repeated in ministers' programme for government.
The goal would require more than 1,850 of the country's 3,700 licensed buses to be converted to zero-emission vehicles, according to Transform Scotland, a national alliance for sustainable transport.
Stuck in traffic report
The organisation's report, stuck in traffic, was published ahead of today's 2022-23 programme for government announcement.
It found there were approximately 280 zero-emission buses operating in Scotland, with funding awarded for another 325.
On this trajectory, more than 600 buses - 16% of the fleet - would be decarbonised by the end of 2023.
Transform Scotland's report analysed 10 headline transport commitments from last year's Programme for Government.
The study said efforts to decarbonise the bus fleet, improve bus priority on Glasgow motorways and introduce fair rail fares had made "no progress" or were unlikely to meet their targets.
It identified "slow progress" on targets relating to free bus travel for under-22s, car traffic reduction, low-emission ferries, decarbonising rail services by 2035 and bus-priority infrastructure investment.
However, it said "good progress" had been made on aims to increase the active travel budget and phase out ICE (internal combustion engine) vehicles.
Missing target
Report author Marie Ferdelman said missing the bus-decarbonisation target would be "hugely disappointing" as she called on the government to step up its efforts.
Transport Scotland said its attempt to meet the bus target had been hampered by the Covid-19 crisis.
A spokesman told the BBC: "We've always been clear that this was an ambitious target that we could not meet alone - that's why the work of our bus-decarbonisation taskforce has been critical to our success so far.
"Additionally, no-one could have anticipated the profound impact of the global pandemic.
"We were right to aim high and to ensure progress across the sector - driving ambitions into delivery, with the proportion of zero-emission buses in Scotland now approximately three times higher than that in England.
"Coupled with the provision of free bus travel entitlement to all under 22s across Scotland - we're putting bus at the front of our green recovery from the pandemic."