Human factors in sight distance: For clear and distinct vision relevant to highway objects/signs, the images should subtend approximately what cone angle at the driver’s eye (retina)?

Introduction / Context:
Perception of roadway objects depends on the retinal image size and contrast. Design of letters, symbols, and sight distances uses the notion of a minimum visual angle (cone) for rapid and reliable recognition by drivers.

Given Data / Assumptions:

• Normal daytime visibility with typical driver eye characteristics.
• Objects/signs sized and placed per standard practice.

Concept / Approach:
Experiments show that for “clear and distinct vision,” highway objects should subtend a minimum visual angle on the retina. A commonly adopted angle for distinct recognition in motion is about 3 degrees, balancing legibility and reaction time at speed.

Step-by-Step Solution:
Relate object size and distance to the visual angle it subtends.Adopt the recommended minimum angle ensuring detection and recognition.Select 3° as the standard guideline value.

Verification / Alternative check:
Sign design charts that fix letter heights against approach speeds inherently target visual angles around this magnitude to ensure legibility at expected distances.

Why Other Options Are Wrong:

• 2°: may be inadequate for quick recognition while driving.
• 4°–6°: acceptable but larger than the baseline “minimum” used in many design checks; not necessary in all cases.

Common Pitfalls:

• Assuming the same angle suffices at night or in rain/fog; adverse conditions require larger, more reflective targets.
• Ignoring driver age; older drivers may need larger angles for clarity.

Final Answer:
3°