Traffic Engineering

Question

An exit ramp sign with 6 words, located 10m
away (to the side) from the driver path.
• Given
– Design acuity (worse than average) = 20/40=0.5
– Design speed = 90km/h.
• Questions
– We design on the safe side:
(1) How far from the exit should the sign be located?
(2) What letter’s height should we use in the sign?

Answer 1

In traffic engineering, the design of road signs takes into account factors like driver acuity, design speed, and sign placement to ensure safe and effective communication with drivers. Let's address your questions:

1. How far from the exit should the sign be located?

To determine the recommended sign placement distance, you can use the "legibility distance" formula. The legibility distance is the distance at which a driver should be able to read and understand the sign based on design speed and driver acuity.

The formula for legibility distance (Ld) is:

Ld = 1.47 * (V^2) / A

Where:

1. Ld = Legibility distance in meters
2. V = Design speed in kilometers per hour (90 km/h in this case)
3. A = Design acuity fraction (0.5 for worse than average)

Plug in the values:

Ld = 1.47 * (90^2) / 0.5 Ld ≈ 24,930 meters

So, the recommended sign placement distance from the exit should be approximately 24,930 meters or 24.93 kilometers.

1. What letter's height should we use in the sign?

The letter height in a sign is determined based on the driver's legibility distance and is typically specified as a minimum. In this case, we've calculated the legibility distance to be approximately 24.93 kilometers.

However, it's important to note that sign letter height is typically measured in inches (or millimeters) rather than meters. To convert from meters to inches, you can use the following conversion factor:

1 meter = 39.37 inches

So, if you want to determine the minimum letter height in inches, you can multiply the legibility distance by 39.37:

Minimum Letter Height (in inches) = Legibility Distance (in meters) * 39.37 Minimum Letter Height ≈ 24,930 meters * 39.37 inches/meter ≈ 981,111.1 inches

Therefore, the minimum recommended letter height for the sign should be approximately 981,111.1 inches. However, this value is impractically large for a road sign.

In practice, road signs have standard letter heights specified by traffic engineering regulations. These regulations vary by country and jurisdiction. You would typically choose a standard letter height that complies with local regulations and is practical for road signage.