Correct. And the onus would be on the defendant to prove the duress as you stated.
Check out this scenario, I don't know the chance of success but it sounds reasonable to use. After all, it all depends on the defendant and the circumstances:
I could see someone saying that they saw a dark looking, unknown vehicle tailgating them too closely at 1 a.m. in the morning with very few (if any) vehicles driving by. The tailgating was too close that it would have been a danger to hit the brake in a highway at that speed.
OP tried to speed a tiny bit hoping to maintain a safe distance from the other unknown car in case of a sudden break due any unforeseen circumstances but to no avail, the unknown, dark vehicle sped up to drive again to stick to the OP's bumper. The OP started to fear for her safety, she notices that the actions of the unknown vehicle leave no doubt that this car is targetting her. She worries that after leaving the club (or restaurant, or wherever she was), maybe she was targetted as a potential prey for rapists. She was worried she'll end up being raped or worse.
At the circumstances, with no doubt that she is being targetted by the unknown vehicle, the only option for her is to try to flee and hope to see a police patrol on the highway or reach a police station. (Who knows whether she had a phone with her to dial 911 or whether she even had a chance to do so).
After speeding up, the unknown vehicle turned its police emergency lights on. She then felt safe to stop as it is a police car and her fears for being raped would now diminish. She then slowed down and stopped at the officer's request.
What do you guys think of this defence?
N.B: I sped up once after being tailgaited by a cop. That was 8 years ago. I was anxious to have that officer on the stand for cross examination but unfortunately he resigned before the trial. So he escaped the exercise 