Apologies, for some reason I overlooked this post.
The cross over curves will most certainly cause a crash.
There are generally 2 schools of thought.
One school is to put them at the beginning of the curve so that people are under maximum braking and once they have cleared the cross over they can accelerate out of the curve.
The other school is to put it at the end as the cars will be at their lowest speed allowing a possibility to brake even more and more importantly more time to react.
For the first school there is the argument that given the now out-of production digital curve changers required a half straight before them to work shows that Scalextric tends to think having them at the beginning of the curve is better.
My personally opinion is a hybrid: If you have the space put an R3 in front of the crossover or even better an R4 followed by an R3 and then the crossover.
Failing that put a 22.5° R2 in front and another after the cross over so that people have time to react and slow down.
Another option is putting a "side swipe" or squeeze track before the curve to force people to slow down.
This requires space for 2 full straights though...
I actually try to avoid the crossovers but they can be fun I do admit!