Going up and coming down: a hill in the words the instruments use

Almost everything a driver is told about a hill is a verb with the noun missing. Use a low gear. Do not ride the brakes. Give way going up. The nouns exist, and they are not ours: they are in traffic codes, in road design standards and in the handbook in the glovebox.

They are worth having, because a hill is where the cost of not knowing the word arrives late — a mistake made at the top is paid for at the bottom, by which time the options have narrowed to whatever somebody else built beside the carriageway. What follows is the vocabulary as the instruments define it, and then one ordinary hill in those words.

The words, as the instruments define them

A steep gradient. Spain’s Reglamento General de Circulación puts a number on it: sections of great gradient are those with a minimum inclination of seven per cent. The American sign manual pairs steepness with length: the Hill sign should be installed in advance of downgrades of 5 per cent for more than 3,000 feet, 6 per cent for more than 2,000, 7 per cent for more than 1,000, 8 per cent for more than 750, or 9 per cent for more than 500. The figure printed on the sign means the same in both systems — the Spanish code says of its descent warning that the figure indicates the gradient as a percentage — and the American manual allows a USE LOW GEAR plaque beneath it.

Engine braking. The clearest definition is a negative one. The Highway Code’s coasting rule — a vehicle travelling in neutral or with the clutch pressed down — says it reduces driver control because engine braking is eliminated, vehicle speed downhill will increase quickly, and increased use of the footbrake can reduce its effectiveness. The commercial driver’s manual states the positive form as an obligation: you must use the braking effect of the engine as the principal way of controlling your speed, and that effect is greatest near the governed rpm with the transmission in the lower gears.

The gear a descent is taken in, chosen at the top. The Highway Code’s rule is one line: select a lower gear before you reach a long downhill slope, which will help to control your speed. The commercial manual gives the cost of leaving it late: do not downshift after your speed has built up, because you will not be able to shift into a lower gear, may not be able to get back into any gear, and all engine braking effect will then be lost. It also retires a piece of folklore: using the same gear down as you would need to climb was a rule for older trucks, and a modern vehicle with less friction and drag to hold it back may need a lower one coming down than going up.

Brake fade. The manual’s definition is the one to hold: if the brakes become too hot they may start to fade, which means you have to apply them harder and harder for the same stopping power, and if you continue to use them hard they can keep fading until you cannot slow down or stop at all. The mechanism is friction turned into heat, in components built to take a lot of it but able to fade or fail from the excess caused by using them too much and not relying on the engine. A highway agency’s escape-ramp guide traces the chain: a driver unfamiliar with a mountain road may descend in too high a gear, requiring increased braking to control speed, which causes properly maintained brakes to overheat and fade and improperly maintained ones to fail. Spain’s traffic authority adds where it bites first: braking excessively down a mountain pass can overload the system and cause a loss of effectiveness through heating, especially on the front axle, given how the weight is distributed. None of the instruments read here used the term vapour lock; they name fade and loss of hydraulic pressure, so those are the words used.

The firm, brief application — not the constant one. Once the vehicle is in the proper low gear, the technique is written down with numbers: brake just hard enough to feel a definite slowdown; when the speed has dropped to about five mph below your safe speed, release, an application of around three seconds; when the speed builds back, repeat. The brakes, the same manual says, are only a supplement to the braking effect of the engine. Spain’s authority reduces a prolonged descent to two premises — an appropriate speed, and moderate use of the brake at all times.

The escape ramp, and the arrester bed inside it. A state design manual lists the family: gravity ramps, ascending side roads and the least desirable design because a vehicle with no braking left can roll back down after stopping; and arrester beds, the most desirable, parallel ramps of smooth, free-draining gravel that stop the vehicle by increasing rolling resistance, the gravel kept loose because the bed works by letting the vehicle sink into it. Spain’s code defines the same object from the driver’s side, as sign S-16 — an escape zone off the carriageway, prepared so that a vehicle can be stopped in the event of a failure of its braking system, its entrance marked by a red-and-white chequer where stopping and parking are forbidden. The instruction that goes with it is unhedged: your best hope is an escape ramp, and any driver who loses their brakes going downhill should use one if it is available.

The hill start and the hold function. A current owner’s handbook describes hill-start assist as holding the brakes automatically for approximately two seconds — up to about five with the accelerator slightly depressed — so the vehicle does not roll back, and then carries the caution that matters more — it does not replace the need to apply the brakes while stopped on an incline. Its descending equivalent, downhill brake control, holds a set speed between roughly 4 and 40 km/h with the driver off the pedal, and the handbook notes that its yellow indicator may mean the system has overheated.

Gross combination weight, and the trailer’s own brakes. The commercial rating is the manufacturer’s value for the power unit, or the sum of the ratings of power unit and towed units, whichever is highest: a ceiling for the combination, not for the vehicle you sit in. The British rules for a car and trailer are more concrete — an unbraked trailer must not weigh over 750 kg nor more than half the car’s weight, a breakaway cable must engage its brakes if it has them or stop it if it detaches, and towing limits are 50 mph on single carriageways, 60 on dual carriageways and motorways.

Priority on a narrow gradient. Spain’s code decides it: on a steep section, where the narrowness described for constricted stretches applies, priority belongs to the vehicle travelling uphill, unless it could reach a pull-in place first. The Highway Code writes the same instinct as courtesy — on single-track roads, give way to road users coming uphill whenever you can, reversing if necessary until you reach a passing place.

Your hill, going up

Going up, gravity slows you, and the steeper the grade, the longer the grade and the heavier the load, the more you must use lower gears to climb it. That is why climbing lanes exist: a heavy vehicle losing speed on a gradient is not being driven badly, and the Highway Code’s rule is to use the lane if you are the slow vehicle or if traffic behind wants to overtake. An overtake begun on a climb is an overtake into a shortening sight distance, a subject of its own in this section.

And coming down

Coming down, the mechanics reverse. Gravity, which was taking energy out of the vehicle on the way up, now puts it in continuously, and the commercial manual states the consequence flatly: in coming down long, steep downgrades, gravity causes the speed of your vehicle to increase. The brakes are the only part of the car whose job is to turn that energy into heat, which is why the vocabulary above exists. No number is attached to it here: it depends on mass, grade and length, and no instrument read here publishes one for a car.

The manual’s list of what sets a safe speed is the most transferable thing in it, and none of its four items is your confidence: the total weight of vehicle and cargo, the length of the grade, its steepness, and the road and weather conditions. Then the sequence is fixed. Pick the low gear at the top, before the speed exists. Let the engine hold the car. Use the brakes in firm, brief applications rather than as constant pressure. And if the pedal is going long and the car is accelerating anyway, stop treating it as a driving problem: the ramp is signed in advance, and the people who build them say to use it.

And when the journey ends on the slope, two codes say one thing: handbrake firmly on, a forward gear facing uphill and reverse facing downhill, with the wheels turned away from the kerb going up and towards it coming down.

What the simulator can, and cannot, show you here

The section’s crash simulator models a flat road, and it is honest to say so first. It has no slope control, so its distances are computed as though the road were level. What it gives you is the baseline a descent makes worse — reaction and braking distance at a given speed on a given surface. Read it as the best case, and the hill as what lengthens it.

What we cannot tell you

We cannot give you a stopping distance for your car on a seven per cent grade: none of the codes or manuals read here publishes one, and computing it would invent a figure with an authority it has not earned. Nor can we tell you which gear your vehicle needs, or who has priority on the narrow climb nearest you — the manual that comes closest says you should know what is right for your vehicle, and the priority rule differs by market and is stated in each market’s own code.

The hill is simple once it has its nouns. It is a percentage and a length; it is taken up in a gear low enough to hold speed and down in one chosen before the speed exists; the brakes supplement the engine, not the other way round; and somebody has already built the thing that stops you if you get it wrong.