The vehicles for the 19 foot-tall people would have to be designed and operated differently than vehicles for humans, of course. But it is probable that your alien planet would probably have lower surface gravity than Earth, and so their vehicles would probably be designed and driven differently on that planet than they would be on Earth.
A 19 foot tall intelligent species would be possible on a planet with Earth like surface gravity.
The tallest recorded giraffe, George, stood 20 feet tall.
The extinct giant ground sloth Megatherium americanum was huge.
Megatherium americanum was one of the largest animals in its habitat, weighing up to 4 t (8,800 lb),5 with a shoulder height of 2.1 m (6 ft 11 in) and length of 6 m (20 ft) from head to tail. It was one of the largest ground sloths, about as big as modern Asian elephants.
Megatherium had a robust skeleton with a large pelvic girdle and a broad muscular tail. Its large size enabled it to feed at heights unreachable by other contemporary herbivores. Rising on its powerful hind legs and using its tail to form a tripod, Megatherium could support its massive body weight while using the curved claws on its long forelegs to pull down branches with the choicest leaves. This sloth, like a modern anteater, walked on the sides of its feet because its claws prevented it from putting them flat on the ground. Although it was primarily a quadruped, its trackways show that it was capable of bipedal locomotion. Biomechanical analysis also suggests it had adaptations to bipedalism.
So that is an example of a sometimes bipedal mammal the size of an elephant that might have reached close to 19 feet tall on its hind legs.
I have seen a photo of a big male Asian elephant walking on his hind legs with a woman standing on his tusks. He would walk on his hind legs for a considerable distance in his performances.
In the wild, big male African elephants often rear up on their hind legs to grab high tree branches with their trunks. A big bull that is about ten feet tall at the shoulder would easily rear up to 15 or 16 feet tall at the top of the head, and could reach over 19 feet high with their trunk.
Since the largest African elephant ever measured was more than 13 feet tall at the shoulder, I assume that if he was able to stand on his hind legs the top of his head would be about 19 feet above the ground.
Of course African elephants don't stand on their hind legs much, nor walk far on their hind legs. But a being somewhat similar to an African elephant but thinner and weighing less might become bipedal.
I note that elephants do make and use tools, though not as much as humans, and their intelligence range might have a considerable overlap with that humans, and they might be classified as intelligent beings and persons instead of mere animals.
Some extinct proboscidean species were larger than modern elephants.
The largest land mammal of all time may have been a proboscidean; Palaeoloxodon namadicus was up to 5.2 m (17.1 ft) at the shoulder and may have weighed up to 22 t (24.3 short tons), almost double the weight of some sauropods like Diplodocus carnegii.4
One partial skeleton found in India in 1905 had thigh bones that likely measured 165 centimetres (5.41 ft) when complete, suggesting a total shoulder height of 4.5 metres (14.8 ft) for this individual elephant.
Two partial thigh bones were found in the 19th century and would have measured 160 cm (5.2 ft) when complete. A fragment (currently unlocated) from the same locality was said to be almost a quarter larger; volumetric analysis then yields a very speculative size estimate of 5.2 metres (17.1 ft) tall at the shoulder and 22 tonnes (24.3 short tons) in body mass, which if correct would make P. namadicus possibly the largest land mammal ever, exceeding even paraceratheres in size. However this estimate based on the "distal femur portion" requires reexamination, as the author himself could only suspect that "fossils are likely stored in the Indian Museum of Kolkata; until such a collection can be revised, this size estimate will remain speculative."
So if the largest size estimate is correct, and if individuals of Palaeoloxodon namadicus were intelligent enough to be considered people, they would examples of people who once lived on Earth who were almost as tall as your 19 feet when walking on all fours.
And it is easy to imagine a proboscidean species which was thinner and taller than Palaeoloxodon namadicus or any other known proboscidean species and reaching a height of 19 feet while on all fours, while not weighing more than about ten tons.
So it is theoretically possible for a bipedal species of intelligent beings to be about 19 feet tall on a planet with the same surface gravity as Earth.
On a planet with a lower surface gravity than Earth, animals and people would have lower weights relative to their body mass. And though 19-foot-tall people should be possible on planets with Earth's surface gravity, I suspect that they would be many times more common on planets which have a much lower surface gravity.
I note that the ability of a planet to retain atmospheric gases, including the oxygen necessary for large land animals and for intelligent beings, depends on the ratio of the planet's escape velocity divided by the speed of gas molecules in the exosphere of the planet.
It is the escape velocity, and not the surface gravity, of a planet which is important to that planet's ability to retain a dense atmosphere for geological eras of time, time enough for intelligent life to develop on it. And the surface gravity and escape velocity of worlds are calculated according to different formulas and don't vary the same way with different planetary properties.
Here are links to online surface gravity and escape velocity calculators that I sometimes use.
According to a calculation on page 54 of Habitable Planets for Man, Stephen H. Dole, 1964, the least massive planet capable of retaining an oxygen rich atmosphere for long would have an escape velocity of 6.25 kilometers per second.
An escape velocity of 6.25 kilometers per second is 0.55873 of Earth's escape velocity of 11.186 kilometers per second.
According to Dole, a planet with an escape velocity of 6.25 kilometers per second would have 0.195 Earth mass, 0.63 Earth radius, and 0.49 of Earth's Surface gravity. Note that the surface gravity and escape velocity don't have the same ratio compared to Earth's surface gravity and escape velocity.
However, Dole's formula for the relationship between the mass of rocky planet and its radius, average density, surface gravity, and escape velocity is probably obsolete due to our greater present astronomical knowledge. There is considerable possible variation in the average density of terrestrial type planets. Thus you might be able to design a planet with a surface gravity lower than 0.49 g and an escape velocity higher than 6.25 kilometers per second.
But there are limits to the density of a terrestrial type planet. Iron is the densest common material in the universe, so no terrestrial type planet should have a higher average density than that of iron. And if a planet has too low a density, its entire surface will be covered with a liquid ocean many miles deep, and there won't be any dry land. Also, the lower the average density of the planet, the rarer metals will be, and the people will need metals for many machines and vehicles.
I assume that the animals and people on your planet will have an average density similar to water, like animals on our planet. Thus your 19 foot tall people will have the same mass they would have on Earth. So their inertia when they stop or start moving would be the same as on Earth.
But their weight would be much less on a low gravity planet than on Earth.
Astronauts in microgravity have to remember when moving large objects than their mass and inertia is the same on Earth, while the weight of the objects is zero.
If there are any characters from Earth in your story, they will find they have to remember that the mass of objects is the same while the weight is lower on your planet.
I note that the materials the machines and vehicles are made out of should be the same as Earth materials, and so should have the same mass and inertia as those objects have on Earth, while being much lighter on that planet.
So the vehicles for the 19 foot-tall people on your planet, if it has a lower surface gravity, should be engineered and driven somewhat differently than they would be on Earth.