How Do Modern Speed Limiters Work?

Speed limiters on late model vehicles work via the computer software instructions that are integrated within the various engine management functions.

The system relies on data received from sensors that supply “road speed” information to the primary computer - thereby signalling the requirement to restrict ‘specific’ engine management performance. These control processes are usually “spark” restriction or fuel restriction, or a combination of both. The result is totally effective for restricting Top Speed without affecting performance in any way. That is, it does not affect engine power – until a predetermined “activation” speed is reached.

NB. Since publishing, another, very informative web article that complements the facts presented here, is <.Tell-them-about-top/speed/limiting-as-well-as-roll/over/prevention... >.  In the article, professor John Banzhaf says, "It would make little sense to focus on electronic systems to reduce roll-overs, and completely ignore another current electronic system which could prevent virtually all high speed accidents".

The speed limiter is in essence a “rev-limiter” and thus only impacts on top speed. To this end, the speed limiter has evidently come into existence primarily as a result of modern technology being used to make a more effective mechanism for "limiting" the over-revving of engines inadvertently- e.g. as might happen in a racing situation. However, as we all know, once technology is employed you can't 'turn-the-clock-back' - and hence, by the early 1990s we suddenly had an effective means of restricting unnecessary top-speeds [as a "by-product" of the technology, for example].

The power factor – i.e. the cars ability to accelerate - to any particular speed - is unaffected and completely independent of this.  The vehicle can continue to be accelerated to a (road) speed specific ‘top-speed’ at which point it will retain its power and performance but cease from further increases.

Note:  This type of system cannot work for vehicles that do not have computer-chip-controlled engine management, otherwise known as EFI, < www.--electronic-fuel-injection-... >.

‘Tampering’ issues
It is virtually impossible to alter or tamper with by system by interfering with the mechanical mechanisms that the ‘electronic’ aspects of the system are connected to. This is because the relationship integrated between these must initiate through the electronic side of the system in the first instance – or the whole system would fail.  It would however be possible for defaulters to circumvent "regulated" speed limiters by down-loading complete ECU [engine computer programmes] from the Internet. However, it is currently illegal to down-load many types of material that is deemed to be harmful. Therefore, with correct legislation in place the down-loading of ECU information could be kept to a minimum - and those caught participating could face hefty penalties.

In other words, if authorities were serious about preventing injuries from excessive speeds they could easily promulgate laws/legislation to restrict and control certain types of (vehicle) ECU software. Naturally this would need to encompass a degree of 'buy-in' and industry regulation from the motor vehicle companies.  Draconian "yes", and slightly impinging on the rights of individuals, to drive at really excessive speeds on public roads! ...and to tune their vehicles accordingly?...
However, in New Zealand and Australia no one was really thrilled about the new (draconian type) legislation needed for passenger air travel after the 911 terror attacks in the USA. But these impingement's, regarding such seemingly innocuous things as carrying small quantities of liquid onto aeroplanes, have nonetheless prevented hundreds of needles deaths that might have occurred had they not been implemented.  It is therefore slightly ridiculous for detractors to argue against top-speed restriction on some basis of perceived 'civil rights' - for actions that are already totally illegal.
Refer also; Emergency Service Vehicles Exempt.  -  Specialist Equipment for AF & Police.

Q; If speed limiters are so effective why are they not being used already?

A; They actually ARE being used – and have been since as far back as 1990 – but the activation speed is (still) being set at 180km/h – and even as great as 230km/h in some instances.  Read Also: Why Do Cars Need To Go So Fast? 

Manufacturers could recalibrate the ECU software, and have the activation at a socially acceptable speed if they wanted to. In fact, in recent years [since 2004] we (the general public) have heard a lot from manufacturers about other electronic safety features – such as electronic stability control (ESC), which is a very useful function in its own right. However, in respect of ESC, the distributors neglect to explain that the very same computer that the ESC function/s is integrated into, also contains an effective speed limiting function.  Read also: ESC and Speed Limiting.

Q; What about the GPS integrated systems – aren't they more effective?

A; These are merely [very sophisticated] radio telemetry integration systems connected with the very same speed limiting software function that this website endorses - and/or that can be adjusted [re-calibrated] in your modern vehicle.
Some governments our (now) experimenting with these GPS [also know as ISA] integrated systems - via “trials” on departmental fleet vehicles.  See also: Oliver Carsten and "Other Info On the Internet".

However, as you can imagine it relies on the GPS network continuing to work reliably and not being tampered with (disrupted). It has the ability to restrict speed in virtually all (identified) speed zones – not just “Top-speed” open highway.  It would be very expensive to implement widely however, and always be reliant on the GPS network
Conversely, the on-board “top-speed only” recalibration method will still work even if all satellites were destroyed by meteors! – and the cell phone network collapsed at the same time.

NB. The greatest percentage of serious injury crashes occur when a vehicle/s is being driven at speeds under 110km/h – but are [nonetheless] too fast for the conditions. Maybe in about 20 years from now, when a reliable and tamper proof GPS integrated speed limiter system is available at low cost, these crashes can be reduced.
In the interim, the 10 to 14 percent of (serious injury) crashes resulting from speeds above 120km/h can be prevented – in the FIRST instance – by using the presently available system.
Note: Many of these (crashed) vehicles have more that one occupant - translating to an [injury] percentage statistic, for New Zealand roads, of around 24 percent.  A similar statistic would most probably apply for Australian roads - though we haven't specifically collated their crash data.

Refer also: "Analogy... Was John Key a Boy-Racer... In 1981?"

A web article containing complementary information about modern speed limiter functions, particularly where competencies around overtaking are concerned, can be found at < http//:idisk-Public/ANCAPsped-limiter%commentpdf >.   The only problem that is evident with that (cited ANCAP) article however, is that it purports to have done research showing that the percentage of fatalities attributable to speeds above 120km/h is only about two percent, of total road fatalities?  We believe this statistic to be patently flawed, for Australian roads, and suggest that any "cursory" study of serious crashes - in both NZ & Australia over past ten years - depicts the relevant percentage to be at least eight-twelve percent; and that is just for 'fatalities' and doesn't take into account the horrendous toll suffered through serious injuries. 
Supporting our assertion regarding this '2% mistake' in the Ancap report, are numerous other road safety articles such as <www.Smart-motorist//excessive-speed-factor > < WWW.---biggest-killer-on-NewSouthWales-roads... > < www.--Advocates-for-Highway-and-Auto/Safety--> < UK-suggests-a-29%-reduction... >   < www.../..//.. > and many, many, more similar sites.
In analysing the crash statistics provided on these sites it should always be remembered that Alcohol related deaths often have, as their initiating cause of (actual) death, a very high and unnecessary speed in the first instance. 
Further Reference: <Health_an_Safety_In_the_work/Place_2002_regulations_nz... > >
NB. Intransigent Ignorance: Unfortunately for many innocent and culpable victims alike, there are still (in 2012) police traffic spokespeople such as Michelle Roberts of WA, who remain ignorant of the ability to simply restrict these unnecessary top speeds. Refer < Roberts calls-for-power-ban-/ >..