With Citroen's C3 Stop & Start, the main questions centre not around why you'd buy one but why you wouldn't want to. After all, the arguments for an automatic engine switch off in stop-start traffic are pretty straightforward.

With the 'Stop & Start' system, the engine is automatically turned off and is in 'standby mode' when the vehicle stops at traffic lights and in traffic jams. The engine instantly starts up again when the brake pedal is released, with the vehicle pulling away once the accelerator is pressed. This C3 was the first mass-produced car equipped with this innovative system and, for the time being, can only be ordered in 1.4-litre 16v petrol form with the semi-automatic 'SensoDrive' transmission, Citroen's electronically controlled manual gearbox.

In testing, Citroen reckon that the 'Stop & Start' system has reduced fuel consumption by 10% for city driving, 6% in a standard combined cycle and up to 15% in heavy traffic. All this sounds reasonable, as does the fact that CO2 emissions should be reduced by a similar proportion. You could also argue - and Citroen do - that widespread adoption of 'Stop & Start' technology would help to improve the quality of life in cities, since drivers and the urban environment would benefit from total silence during the many times vehicles are stopped.

With studies slowing that traffic in inner city areas can be stationary for up to 30% of the time, this C3 could offer even greater savings for urban drivers faced with the daily prospect of traffic lights, pedestrian crossings and congestion. The system stops the engine a split second before the car comes to a standstill, then restarts it again in just 400 milliseconds (twice as quickly as a traditional starter motor) the moment the driver takes their foot off the brake pedal and wants to accelerate away as normal.

The secret behind this technology is apparently a reversible alternator that acts as a standard alternator one minute (driven by the engine to charge the battery) but when required, also acts as a starter motor, driven by the battery to effortlessly restart the engine.

"With traffic in urban areas stationary for up to 30% of the time, 'Stop & Start' makes sense."

Since 'Stop & Start' is the kind of technology aimed at people who've one eye on fuel bills, you might have expected it to appear on a diesel-engined model first. It doubtless soon will but, for the time being, Citroen are looking for a publicity splash and the simple truth is that the savings that this system achieves allied to diesel power are nothing like as great.

Perhaps more significant is the fact that 'Stop & Start', like the 1.4-litre 16v petrol model itself, can't be ordered with a conventional manual gearbox, instead working as part of the semi-automatic SensoDrive paddle shift system. Since incorporating it means only a few changes to the software, Citroen are consequently able to offer the technology at little or no extra cost for C3 1.4 16v petrol customers.

Enough on 'Stop & Start', what about the 1.4-litre engine under the bonnet? Well, performance is brisk without risk of turning the front tyres into smouldering carcasses every time you tread on the righthand pedal. The C3 1.4 16v will accelerate to 60mph in 12.7 seconds before running on to a very respectable top speed of 111mph. The extra 11 lb/ft of low and mid range torque make the 16v unit feel a good deal gutsier than its 8v sibling. Because the engine also features a variable valve timing system, you'll feel the benefit of that torque across a wider range, making this C3 happier when pulling from a standstill.

It's also a good deal more refined than the 8v unit when extended. You won't even pay a penalty in terms of overall fuel consumption, the 16v engine returning an average of 47.1mpg which is even better than the 8v unit. The 143g/km of carbon dioxide emissions also land this particular C3 in a very low taxation band.

And the Sensodrive gearbox? Well, it does make this C3 supremely easy to pilot in urban traffic. The gearbox features a mode whereby it defaults to a conventional automatic transmission, thus saving you the bother of shifting yourself. When the road opens up, you can switch it to sequential mode and flip up and down the gears using paddles behind the steering wheel or the centrally mounted gear lever. It's not one of those clever clutchless manual systems - there's still a conventional automatic torque converter - but it does offer three different programs (Normal, Sport & Snow) as well as that fully automatic mode. Also worthy of note is that fact that SensoDrive has been tweaked in the latest facelifted C3 models. It now features revised shifting paddles with a higher-quality feel and a chrome-topped gearlever that slides around its gate rather than centring itself after a mode has been selected.

This version comes only in Rhythm trim, which means you can expect to find luxury items like lateral airbags, fog lights, body-coloured front and rear bumpers and side rubbing strips, plush interior trim and the option of a full size electric glass sunroof, perfect for those sunny summer days.

The technology behind 'Stop & Start' is nothing new. Volkswagen dabbled with it twenty years ago. However, Citroen was the first manufacturer to bring it to the volume market. You have to wonder what has taken everybody so long - and how long it will be before it's simply something we all take for granted.