What is the relationship between speed limits and sight distance?

What is the relationship between speed limits and sight distance?

• A. Higher design speeds require longer sight distances to maintain safe stopping and decision distances.
• B. Higher speeds require shorter sight distances
• C. Sight distance is independent of speed
• D. Sight distance only affects pedestrians

Answer: (A)

The key idea is that the distance you need to safely respond to a hazard grows as speed increases. You must have enough distance to see the hazard, decide what to do, and actually stop. The part of the distance you cover while you perceive and react to a hazard grows linearly with speed (you travel more feet in the same reaction time). The braking portion grows even faster, roughly with the square of speed, because stopping from a higher speed requires much more distance under deceleration. Put together, higher design speeds demand longer sight distances to maintain safe stopping and decision distances. So, as speed goes up, the required sight distance goes up, not down. Sight distance isn’t independent of speed, and it applies to all road users, not just pedestrians.

The key idea is that the distance you need to safely respond to a hazard grows as speed increases. You must have enough distance to see the hazard, decide what to do, and actually stop. The part of the distance you cover while you perceive and react to a hazard grows linearly with speed (you travel more feet in the same reaction time). The braking portion grows even faster, roughly with the square of speed, because stopping from a higher speed requires much more distance under deceleration. Put together, higher design speeds demand longer sight distances to maintain safe stopping and decision distances.

So, as speed goes up, the required sight distance goes up, not down. Sight distance isn’t independent of speed, and it applies to all road users, not just pedestrians.