Answer: If you want to calculate whether there is danger, you can draw a circle with point C as the center and 250 meters as the radius. If the AB Expressway is out of the circle, it is safe and does not need to be temporarily blocked. If it is tangent or intersecting, it needs to be shaded.

Then we can make CD perpendicular to AB and D perpendicular to feet.

The area of the triangle ABC =1/2ac * BC =1/2ab * CD.

Because AC = 300m, BC = 400m, AB = 500m.

So: CD = AC * BC/AB = 300 * 400/500 = 240m.

240 & lt250

Therefore, the AB section of the expressway is dangerous and needs to be temporarily blocked.