Diesel as a fuel has become the staple of car fleets since the government introduced CO2 emission based benefit in kind tax rules in 2002.

Little wonder: it’s thermally efficient – producing low levels of CO2 and plenty of miles to the gallon. Manufacturers have responded with ever more efficient engines featuring incredibly smooth operating characteristics unknown to those rattling, agricultural units of the eighties when diesel units first started to appear in numbers on the market.  No surprise, then, that diesel has become the darling of the fleet world.

But it’s position is being undermined.

And it’s being undermined by NOx.

NOx? That’s the nitrogen oxides (nitric oxide and nitrogen dioxide) emitted during combustion. Because, while diesels are brilliant for low CO2 emissions, they are guilty of NOx emissions (think of petrol cars being the other way around). And NOx has been fingered as a key pollutant associated with respiratory diseases.

The London Mayor’s office has reported that nitrogen dioxide (NO2) was responsible for 5900 deaths in London; if combined with particulate matter (PM), another pollutant from diesel engines, then that figures rise to 9400 deaths.

Little surprise, then, that we’re seeing headlines late last year that major cities around the world were considering a diesel ban by 2025. These included Paris, Madrid, Mexico City and Athens.

Where next? Should alarm bells be ringing for those fleet managers and company car drivers of diesel cars?

I certainly think it would be wise for fleet managers to be on yellow alert.

Let me be clear, though, that the diesel engine remains central to many fleet policies and should continue to do so (depending on use).

I put my hands up: I’m driving an Audi A4 Avant diesel which I love; it is refined; spacious; beautifully built; and marvellously fuel efficient (around 55mpg average).

But it’s the use bit that is key.

If your drivers are regular users of the UK’s motorways racking up 20 to 30,000 miles a year, then there is little to touch a diesel for fuel efficiency: hybrids certainly won’t, neither plug-in hybrids, and with their current range, electric cars are impractical for such epic mileage.

And because motorways aren’t urban centres, then the pollution and respiratory risks dissipate. In the future, if a meeting is in a city, then driverless pods should be able to meet you, such as those developed by the RDM Group in Coventry.

But while all this is true, fleet managers need to start considering alternatives. Do you really want to use two modes of transport when one might do? What are the implications for your business if it is not considered ‘green’ because of your diesel car use? What technology is available to your fleet?

This year, we will see more and more ‘green’ cars suitable for fleet use: electric cars with long-life batteries; plug-in hybrid versions that deliver on both motorway and in the city on pure electric power. These are clearly trends that need focus now in order to formulate future fleet policy.

The government has tacitly admitted that it needed to revise the benefit in kind tax tables to encourage use of such cars by substantially changing the incentives to choose Ultra Low Emission Vehicles (ULEVs) in the revised 2020/21 company car tax rules.

Little help for an already committed electric driver who will have witnessed their benefit in kind roller coaster from 0% in 2014/15 to 16% in 2019/20 before diving back down to 2% in 2020/21. Hardly consistent from HMRC, but at least the government has recognised the issue. And reacted.

it is the tax year 2020/21 where fleet managers should be looking to formulate their fleet policies. Because we are undoubtedly at a pivotal point in which technologies to use for fleets – so the questions to ask during this period of great change are: could ULEVs work for my fleet? What are the costs? What are the implementation strategies?

And how will I react to NO₂ becoming more of a concern than CO₂ ?

Not easy questions to answer. But questions that will need answers.