Flash from the past

This is a picture of funny cars at the Orange County International Raceway in Southern California. I took this in the early 80’s with a medium format film camera, the Mamiya RB 67. The shot was obviously taken at night. I was seated in the top row of the bleachers quite a distance from the track. My flash would have no effect because the distance was too great for the light to reach the cars. The only thing I could do was use the lighting from the other photographers positioned at ground level at the edge of the track. Each of them had flash units. I guestimated their guide numbers at 100 ISO to be 120 -- standard strength for flashes at the time. The formula I used to determine the appropriate f/stop was: GN = f/stop x distance. That is, the guide number equals the f/stop times the flash-subject distance. I guessed the position of the photographers to be about 15 feet on a perpendicular line to the cars. Doing the math, the f/stop = 120 ÷ 15 which equals f/8. So, I set my shutter speed to one full second, and it turned out my f/8 aperture was correct in providing a perfect exposure. Note that the flash froze the cars in action, but the long exposure superimposed the streaks of sparks over the whole scene. Even though this is a single exposure, it has the visual effect of being a double exposure.