Split/Second

This secret achievement is:

Fearless
Overtake 5 trucks in a row during Sudden Death in a Survival event.

Do it in freeplay, lest you run out of target score to achieve and end your run early.

Super easy on the spillway, the first survival track. Just like the other achievement called Convoy, as the wide nature of the stage and easy sweeping turns makes it easy to dodge. You can even hang back and wait for the timer to run out, and then pass the starting trucks to make it a little bit quicker.

This is quite doable with the right car and the right track in Quick play.
I used the Hanzo FX350 as it is the fastest steady car (non auto-drifting) available. It also has very sharp handling, which is perfect for avoiding barrels.
Then just pick Storm Drain and start racing. Try to pass the trucks with ample manouvering space, it's not worth passing them when you don't have space to dodge barrels. Also, it's easier to pass then on the straights and with those small walls between you and the truck, just to be safe.
I got this on my first try without any tricks, together with the Convoy achievement.

This is a very easy, albeit tedious solution. From the Quick Match section choose the Storm Drain map and use a car with high acceleration. Once the game has started:
1) stay close to the first truck all the time and pass it right away only when the time is up.
2) Important once you have passed, stop immediately and be overtaken by 6 other trucks
3) when the seventh truck has overtaken you .. accelerate immediately and pass it, you will see that it will count as a second overtaking.

REPEAT ALL THIS PROCEDURE FOR ANOTHER 3 TIMES AND YOU WILL UNLOCK THE ACHIEVEMENT

Thought I should add this solution that came to my head.

This achievement can be a bit tricky especially for me since, I'm just alright at racing games.

Achievement can be obtained in the Survival mode (One with the trucks dropping barrels from behind), when the time has run out Sudden Death will be activated and the trucks will be dropping red barrels only, this is when you need to overtake to go for this achievement.

Pick Quick Play from the main menu and pick Survival mode (This mode has to be unlocked in the Season mode). Choose the first track as this track is the easiest to try for this and the car to pick is the Hanzo FX350 since this car is quite strong and doesn't automatically drift.

If like me this is the last truck related achievement to get. As soon as the game starts, you want to accelerate to the back of the truck fast, ideally at the back right or left side and stay there till the main time runs out and Sudden Death starts so you can attempt this as soon as.

You want to be behind the first truck, you know if your in the right spot when the truck doesn't drop any barrels, then when it becomes Sudden Death, overtake the truck for your 1 of 5 needed right of the bat.

Also when you are waiting behind tailing the truck, the second truck you can't see doesn't seen to drop the barrels or not as much, this is good cause the other cars that are there will not be destroyed, you will need them as soon as Sudden Death starts and you've overtaken the first truck

Once you have overtaken the first truck hopefully 3 or 4 cars are there when you catch up (With every truck needed to overtake you want to have some of the other cars there with you each time). Look at the cars and pay attention to the route they take. Some cars will hit the barrels and blow up clearing a path for you to get through (Do not rush yourself or panic, take your time to get through and catch up to the other trucks). Other cars will sometimes show you the safe pathway to take or they can be your shield to some of the explosions.

The last bit of advice is mostly go diagonally, meaning if you are at back right then go left past the side of the truck (Do not do this with the first truck). Go slowish so you can aim for the gaps to get through the barrels.

As soon as I started to use the cars the way that I explained I managed to get this done.

That's all guys, I hope this can be of some use :)