Código y gestión del MicroBlaze

Advance warning signs forewarn drivers that they are approaching a signalized intersection. Figure 2-1 shows two types of warning signs. Figure 2-1a shows a sign that uses a “signal ahead” symbolic message. Flashing beacons sometimes accompany this sign to ensure drivers detect and interpret the sign’s meaning. Figure 2-1b shows a “Be Prepared to Stop When Flashing” sign. This sign has the beacons flashing only during the last few seconds of green. It is sometimes referred to as an “advance warning sign with active flashers.” In this mode, the flashing indicates when the signal indication is about to change from green to yellow. When flashing beacons accompany these advance warning signs, they are also named advance warning flashers (AWF). The purpose of AWF is to forewarn the driver when a traffic signal on his/her approach is about to change to the yellow and then the red phase. An effective AWF implementation is intended to minimize the number of vehicles in the dilemma zone during the change interval. In North America, there are three general types of advanced warning devices and the decision of which to use is based on engineering judgment. These AWFs include:

• Prepare to stop when flashing (PTSWF)—A warning sign, BE PREPARED TO STOP with two yellow flashers that begins to flash a few seconds before the onset of the yellow and continue to flash throughout the red phase. A WHEN FLASHING plaque is recommended in addition to the sign.

• Flashing symbolic signal ahead (FSSA)—Similar to previous type except the wording on the sign is replaced by a schematic of a traffic signal. The flashers operate as above.

• Continuous flashing symbolic signal ahead (CFSSA)—The sign displays a schematic of a traffic-signal symbol but in this case, the flashers operate continuously (i.e. they are not connected to the signal controller).

Figure 2-1: Advance warning sign and advance warning flashers

The location and timing of AWF are key considerations for the sign installation. The distance from AWF location to a signalized intersection must be equal to or greater than that required to perceive and react to the flasher and stop the vehicle safely. The timing refers to the length of time before the yellow interval of the downstream-signalized intersection at which the AWF starts flashing. Sayed et al. (1999) indicated that engineering judgment is often the principal guide for AWF installation according their literature findings. However, they also introduced practical guidelines for AWF

implementation used in British Columbia, which are recommended at provincial intersection s where one of the following conditions is satisfied:

• The posted speed limit on the roadway is 70 km/h or greater,

• The view of the traffic signals is obstructed because of vertical or horizontal alignment (regardless of he speed limit) so that a safe stopping distance not available,

• There is a grade in the approach to the intersection that requires more than the normal braking effort, or

• Drivers are exposed to many kilometers of high-speed driving (regardless of posted speed limit) and encounter the first traffic signal in a developed community.

Location of AWFs is calculated by the following equation:

D VT V g f G = + ± 2 2 ( ) Where

V = 85th percentile operating speed or posted speed limit (m/s) T = reaction time (1.0 s)

g = gravitational acceleration (9.81 m/s2) f = friction factor for wet surfaces, and G = grade (m/100m)

The length of the advanced warning time before the yellow interval of the downstream- signalized intersection at which the AWF starts flashing is calculated by the following equation: AW D D V p = + Where

AW = advanced warning time

D = Distance between the AWF and the signal’s stop line

D_p = Minimum distance at which the flashers can be perceived (21.3m)

Studying drivers’ reactions to advance warning flashers in the field is highly problematic because these devices are relatively uncommon and because it is difficult or impossible to establish a controlled experimental environment in which variable parameters can be tested individually. Smith (2001) employed the Human Factors Research Lab’s driving simulator to investigate effects of Advance Warning Flashers at signalized intersections on simulated driving performance. After analysis of the large volume of experimental data, the researchers concluded that AWFs often improve stopping behavior at suitable intersections. But as is often seen in human factors research, human response to a complex situation is not as simple as a linear relationship. In this case, variability in human response resulted in some drivers making a more aggressive—and risky— decision to proceed through the intersection. This finding has obvious implications for field implementation of advance warning flashers at dangerous intersections (Smith, 2001).

Sayed et al. (1999) utilized and analyzed data from British Columbia using two different methods. Models were used to develop expected accident rates at 106 signalized intersections for total, severe and rear-end accidents. Twenty-five of these intersections had AWFs. Although the results indicate that intersections with AWFs have a lower frequency of accidents, the difference between those with AWFs and those without is not statistically significant. An additional before-and-after study was performed for the 25 intersections equipped with AWFs to estimate the accident reduction specific to each location and its approach volumes. A correlation was found between the magnitude of the minor approach traffic volumes and the accident reduction capacity of AWFs, showing that AWF benefits exist at locations with moderate to high minor approach traffic volumes (minor street AADT of 13,000 or greater).