Danger Signs:
What are they: They are PR jump stunts where you either launch your vehicle over a cliff or through a ramp and have to fly a certain distance and land correctly to get 1-3 stars. They are signified with an exclamation mark "!" as a road sign on the map. As you complete Danger Sign PR stunts, more such stunts will unlock and your Danger Sign Hero level will increase.
How to find them: On the map, press
(Change Filter) to sort the map. Deselect all and then select "Danger Signs" to see Danger Signs PR stunts only on the map, signified by an exclamation mark. As you complete Danger Sign PR stunts, more such stunts will unlock and your Danger Sign Hero level will increase.
How to get easy three stars: Your best bet is to get a super fast hypercar that accelerates quickly. I recommend getting a 2011 Bugatti Veyron Super Sport as it will be required for another achievement:
Instead of buying it stock from Autoshow, try to look for it in the auction house. You can easily get a tuned Veyron for 2,000,000 as there are a lot of them available at the time of writing this and many are left unsold with no bids. Specifically, Maka91's tune is brilliant for this purpose, but most tunes that make it an X-999 rated car is a good choice. It is also a good bet as you can later sell it for the same amount or more.
Next, for launch through a ramp or a cliff, make a good long straight run-up to the ramp/cliff and jump it at your highest possible speed. With a Veyron Super Sport, most of them would be easy 3-stars. It would be helpful to set a way-point to the danger sign and follow the guiding line to avoid bumps or spins. Use "rewind" generously if you fail for some reason. In some cases, the landing zone may be in a tricky place where the car will crash on to a house or a railway carriage or something like that, leading to "Failed". In such cases, try to brake the car immediately upon landing or correct your landing by rewinding and attempting the jump again.
Here is a video (not made by me) for completing all 18 Danger Signs:
Additional Notes:For the danger sign at Ambleside Edge, do it with a tuned Mercedes Benz Unimog truck (available for as little as 200,000 CR in auction house at the time of writing this) to attain another achievement called "Ground Force":
There are 18 Danger Sign PR stunts and you don't have to 3-star all of them to get Pilot's License achievement. You need to get 3-stars on 10 Danger Signs and 2-stars on remaining 8 Danger Signs to get to level 10 and complete the achievement. You will eventually want to 3-star every danger sign PR stunt to get this achievement:
Good Luck
