It further recommends that 12-in le

It further recommends that 12-in lenses should be used :
- Where 85th percentile approach speeds exceed 40 mi/h
- Where the traffic control signal might be unexpected
- On all approaches without curbs and gutters where only post-mounted signals are used.
- Where there is a significant percentage of elderly drivers.

The red signals lens must be the same size or larger than other lenses. Thus, a 12-in red lens can be used in conjunction with 8-in green and yellow lenses. However, where green and yellow lenses are 12 inches, the red lens must also be 12 inches.
Table 4.3 shows the minimum visibility distances required for signal faces. A minimum of two signal faces must be provided for the major movement on each approach, even if the major movement is a turning movement. The requirement provides some measure of redundancy of an unexpected bulb failure.
Where the minimum visibility distances of table 4.3 cannot be provided, 12-in lenses must be used, and placement of appropriate “signal ahead” warning signs is required, the warning signs may be supplemented by a “hazard identification beacon.”
The arrangement of lenses on a signal face is also limited to approved sequences. In general, the red ball must be at the top of a vertical signal face or at the left of a horizontal signal face, followed by a yellow and green. Where arrow indications are on the same signal face as ball indications, they are located on the bottom of a vertical display or right of a horizontal display.
Figure 4.19 illustrates the preferred placement of signal faces. At the least one of the two required signal faces for the major movement must be located between 40 and the 150 ft of the stop line, unless the physical design of the intersection prevents it. Horizontal placement should be within 20°of the centerline of the approach, facing straight ahead.
Figure 4.20 illustrates the standard for vertical placement of the signal faces that are between 40-53 ft from the stop line. The standard prescribes the maximum height of the top of the height of the top of the signal housing above the pavement.
Operational restrictions
Continuous operation of traffic control signals is critical for safety. No signal face should ever be “dark” (i.e., with no lens illuminated). In cases where signalization is not deemed necessary at night, signals must be operated in the flashing mode (“yellow” for one street and “red” for the other). Signal operations must also be designed to allow flashing operation to be maintained even when the signal controller is undergoing maintenance or replacement.
When being installed, signal faces should be bagged and turned to make it obvious to drivers that they are not in operation. Signals should be made operational as soon as possible after installation – again, to minimize possible confusion to drivers.
Bulb maintenance is a critical part of safe signal operation, as a burned-out bulb can make a signal face flow. It is followed by a red arrow where the movement must stop.
The MUTCD provides additional detailed discussion on how and when to apply various sequences and combinations of indications.
Signal faces and visibility requirements.
In general, asignal face should have three to five signal lenses (see figure 4.18) with some exceptions allowing for a sixth to be shown. Two lens sizes are provided for :
8-in diameter and 12-in lenses. The manual requires that 12-in lenses be used :
- Where the road users view both traffic control and lane-use control signal heads simultaneously
- Where the nearest signal faces is between 120 ft and 150 ft beyond the stop line, unless a near side supplemental signal face is provided.
- Where the signal faces are located more than 150 ft from the stop line.
- Where minimum sight distances (see table 4.3) cannot be met.
- For all arrow signal indications.