Cruise Control Holds a Floor Speed, a Limiter a Ceiling, and ISA Is EU Law

Modern cruise control was born in 1948 because a blind engineer got tired of his chauffeur slowing down and speeding up whenever he talked. Ralph Teetor was so frustrated by it that he set out to build a machine that would hold the speed instead.

The idea has grown into several different systems that people often lump together: classic cruise control, adaptive cruise control, the speed limiter and the newer intelligent speed assistance now written into European law.

How does cruise control work?

Known variously as speed control, cruise command, autocruise or tempomat, cruise control is a servomechanism: a small control loop that borrows the throttle to hold a speed the driver has chosen. You accelerate to the speed you want, press a button, and the system remembers it. Only adaptive systems skip that first step.

To know how fast the car is going, the system reads a signal from somewhere on the drivetrain. It might come from a rotating driveshaft, a speedometer cable, a wheel speed sensor, the engine's rpm or electronic speed pulses made by the car itself. Most systems refuse to engage below a floor of roughly 25 to 30 mph (40 to 48 km/h).

To act on that signal, the car needs something that moves the throttle. Older designs pull the throttle cable with a solenoid or a vacuum-driven servo. In cars with a drive-by-wire throttle the whole job is done electronically, and on the latest vehicles with electronic throttle control it is built into the engine management system.

Every system has to switch off explicitly and also automatically when the driver touches the brake pedal, and often the clutch as well. Most include a memory function that returns to the set speed after you have braked, and a coast function that lowers it without touching the brake. The throttle pedal still works while cruise is on: press it and the car accelerates, release it and the car settles back to the set speed.

Manual gearboxes make life harder. Pressing the clutch and changing gear usually cancels cruise, and the driver must hit resume after every shift. That is why cruise earns its keep at motorway speeds, where the car lives in top gear, although on many newer manuals the system pauses only briefly while the clutch is pressed.

Cruise control sets a floor that the throttle works to hold; a limiter sets a ceiling that the throttle eases under.

Where did cruise control come from?

Speed control is older than the engine management it now lives in. The Wilson-Pilcher of the early 1900s had a steering-column lever to set the engine speed to be held. In 1908 Peerless fitted a flyball governor with an extra throttle lever on the wheel and boasted that it would maintain speed whether uphill or down.

The governor itself goes back further: James Watt and Matthew Boulton used one on steam engines in 1788, and the device dates to at least the 17th century. It uses centrifugal force to alter the throttle to suit the load, such as a hill.

Teetor patented his version in 1950, a dashboard speed selector wired to the driveshaft and a device that could push back against the gas pedal. He added a lock that held the speed until the driver tapped the brake or switched the system off. A bigger spur than annoyance, though, was the 35 mph (56 km/h) limit the United States imposed during the Second World War to save gasoline and tyre wear, which created demand for something that would keep a car at a set pace.

Others got there too. M-Sgt Frank J. Riley conceived a constant speed regulator while driving the Pennsylvania Turnpike and fitted it to his car in 1948, filing a patent in 1950 that was granted in 1955. Harold Exline, working independently, filed in 1951 for a vacuum powered throttle control with an electrically controlled air valve, granted in 1956. Neither they nor later patent holders were able to collect royalties.

The first production car to carry Teetor's speedostat was the 1958 Chrysler Imperial, which called it auto-pilot. Ground speed came from the spinning speedometer cable, and an electric motor with a two-way screw drive opened and closed the throttle. Cadillac soon renamed the device cruise control. In 1965 American Motors introduced a cheaper cruise command for its large automatics, with a dashboard push button and a vacuum control fed from the speedometer cable. It shut off whenever the brakes were applied.

The electronic era began in 1968 when Daniel Aaron Wisner, an engineer at RCA in Plymouth, Michigan, filed two patents for an automotive electronic cruise control, the second adding digital memory. It was the first electronic device to control a car. Motorola commercialised an integrated circuit for the design in the late 1980s as the MC14460 Automotive Speed Control Processor. The benefit over mechanical rivals was that electronic control could be linked with accident avoidance and engine management.

Does cruise control save fuel?

The 1973 oil crisis is the moment cruise control became popular in the United States, and the sales line of the day was that it saved gas by smoothing out the surges that waste fuel when speed should be steady. By 1974 AMC, GM and Chrysler were pricing the option at $60 to $70, with Ford asking $103.

Increased fuel efficiency is still listed among the benefits today, alongside reduced fatigue on long drives and a way of avoiding the gradual, unnoticed creep in speed that some drivers show on a long highway journey.

The logic is that a servo holds a number while a human drifts. Whether it pays off on a given road depends on the road, and the hill is where the system is most limited. Cruise control manages the throttle, and only the throttle. It will not apply the brakes if the car runs faster than the set speed down a slope, and it cannot stop the car going faster than the chosen speed, even with the engine just idling.

What is adaptive cruise control?

Adaptive cruise control is a general term for a smarter version. Some systems use GPS to find the position of speed limit signs in a database, and many add cameras, lasers and millimetre-wave radar to measure how close the car is to others on the road. Set a following distance and the car slows automatically for vehicles in front, then accelerates back to the preset speed when traffic allows. Sensors can be radar, lidar, camera or a combination.

The limits are worth knowing. Certain systems cannot pick out cars that are completely stationary, or pedestrians, which is why the driver must stay alert. Forward collision warning, which alerts the driver if a vehicle ahead gets too close given both speeds, often comes with it. SAE International classes a car with adaptive cruise as a Level 1 autonomous vehicle.

Cruise control vs speed limiter: what is the difference?

People mix up the two, but they do opposite jobs. A speed limiter is a governor that caps top speed. Cruise control sets a floor that the car works to maintain, using the throttle to push the speed up, and the brake pedal overrides it. A limiter sets a ceiling, eases the throttle to keep under it, and a hard press of the accelerator (kickdown) overrides it. In an automatic, a limiter may drop down through the gears to hold the cap.

Some cruise systems include a limiter function, which will not let the car accelerate beyond a preset maximum, and the limiter has no trouble with the clutch that disturbs cruise in manual cars.

Limiters come in three flavours. Statutory ones are required by law for certain classes of vehicle. Programmable ones are set by the driver to any speed they like and are fitted or optional on models from Citroen, Renault and Tesla, plus certain Fords and Nissans. Fixed ones are built in by the maker.

Manufacturer limits have their own logic. Small commercial vehicles are often held well below car levels, with UK settings of 56, 62, 68 or 70 mph (90, 100, 109 or 113 km/h), to cut fuel bills, maintenance, insurance and speeding fines, with a sticker on the back stating the maximum. Some retailers turn their trucks' limiters down further, to 85 or 80 km/h (53 or 50 mph), saying it trims fuel use and emissions. Many electric cars cap their speed to avoid the rapid loss of charge and range that high speed causes, among them the Smart ED, the Mitsubishi MiEV and the Citroen Berlingo EV, which is held to 90 km/h. Some small economy cars carry limiters for stability and safety reasons, and to spare tiny engines the strain of long overrevving; the first-generation Smart stopped at 135 km/h (84 mph).

The Bugatti Chiron shows the other extreme. Insert the owner's key and the car runs a brief diagnostic, then, if its computer approves the conditions, allows speeds up to 420 km/h (approx. 261 mph) instead of the usual 380 km/h. Bugatti calls it Top Speed Mode, and it lowers the ride height and the rear spoiler to cut drag.

Where limits are voluntary, enforcement is soft. BMW, Mercedes-Benz and others may unhook their limited cars in Europe, Mercedes-Benz supplies some US cars without limiters for an extra price, and third-party companies re-flash the software to remove them.

What is intelligent speed assistance?

Intelligent speed assistance, or ISA, covers any system that keeps a vehicle under a safe or legally enforced speed. It either alerts the driver or reduces speed automatically. It needs to know the local limit, which it gets from the car's position matched against known limits, or by reading road signs.

Passive ISA warns the driver with visual or audible cues, sometimes with a vibration or a stiffer accelerator pedal, and lets the driver decide what to do. Active ISA slows the car to the limit. Most active systems can be overridden, which is thought to help acceptance and safety but leaves plenty of speeding unchecked.

Four technologies are available for finding the limit and the car's speed. Position-based systems use GPS, which has 31 satellites with at least five visible at any time. Radio beacons transmit limits to a receiver in the car and are common for factory sites and logistics centres where very low speeds are needed near workers. Optical recognition reads speed signs, road markings and roadside objects, then uses that limit until it sees a different sign. Dead reckoning estimates the path from wheel rotation and sensors, and it accumulates errors from changes in tyre circumference and from electromagnetic interference.

ISA was born in France, where Saad and Malaterre studied drivers with an in-car limiter in 1982, though drivers set the limiter themselves, rather like a cruise control. Research resumed in Sweden in the early 1990s, culminating in a large-scale trial from 1999 to 2001 with close to 5000 ISA-equipped vehicles. A few hundred of them used a haptic throttle, which became stiffer when the limit was exceeded, with a kick-down function to overcome it.

Who benefits and who objects?

Studies point to safety gains. A 2012 British study estimated ISA could cut road fatalities by half. A 2010 trial in New South Wales showed better compliance with speed zones and less time spent speeding. An Australian cost-benefit analysis from the Centre for Automotive Safety Research in April 2010 suggested injuries would fall by 7.7% with advisory ISA and 15.1% with supportive ISA. A limiting system would cut them by 26.4%, saving $3.7 billion.

The objections are real. A system that follows the posted limit may encourage driving to the limit rather than to the conditions, since curves and gradients can demand less. Some traffic safety practitioners call active ISA an example of hard automation, and some studies found drivers drive up to the limit of the system, where in manual control they have slowed down. Other drivers report giving the road more of their attention once the speedometer needs no watching. Manufacturers have voiced concern that limiters take control away from the driver, and the technology has been compared with speed cameras as something that can alienate drivers. Without speed signs the system can fail, a particular problem when leaving a side road onto a main road, and crossing between metric and imperial countries is a further challenge.

What do the new European rules require?

Under EU regulation, an ISA system must contain a speed limit information function and either a speed limit warning function or a speed control function. The driver must be able to switch it off, it must not prevent the driver from exceeding the prompted speed, and its error rate should be minimised in real driving.

After a political fight, with the UK transport secretary of the time opposing the proposal in 2013, the EU agreed in 2019 to require it on all new cars from July 2022. The EU regulation entered into force on 8 July 2022, and the UK followed with its own speed limiter ISA law on 6 July, a different day because of Brexit. The testing framework is set out in regulation 2021/1958 from 23 June 2021.

The regulation has a consequence for buyers. Models that cannot be redesigned to accommodate the sensors may disappear, and Toyota's GR86 is due to leave European showrooms after 2024, its roofline having no room for an ISA sensor. Commercial use is widespread in Australia, and buyers in Europe can specify ISA on the 2022 Volkswagen Amarok.

When should you not use cruise control?

Cruise control is a tool for steady roads. Michigan warns against using it on ice or snow, and British Columbia in Canada advises leaving it off on wet roads. In some older cars, a skid with cruise engaged means the car keeps accelerating.

If a car is sliding on ice, the advice is neither to brake nor to accelerate, but to let it slow on its own. Avoid it on curves that need slowing, and on rough or loose surfaces. The quieter risk is attention. With feet at rest, a driver can lose spatial perception and stab the accelerator rather than the brake in an emergency.