For large commercial vehicles, the National Safety Council now recommends a minimum 6-s following distance under ideal conditions, in contrast to a 4-second rule used in the past (National Safety Council, 2012). These rules make good sense as guidance. However, it is unknown how effective they are in heavy traffic, where cut-ins occur when gaps between vehicles are large. Cut-ins are important because they can be crash inducing events. One use the 2 and 3 second rules as a basis for determining how long it is safe to look away from the road. Rockwell’s 2-Second Rule (1988) Rockwell (Rockwell, 1988) studied driver "in car" visual performance in 3 different experiments over a 6-year period involving 106 subjects participating in 200 test sessions. The sample was even divided between younger (less then 35 years old) and subjects (over age 45). Data were collected on expressways in light to “mature” moderate traffic. Many of the studies involved glances to AM/FM radios that “stereos,” were being sold in the 1980s, as well as glances to mirrors (Figures 7 and 8). Note that the mean duration for glances to the mirror was 1.1 s, vs. 1.44 s to the radio, with the standard deviation of glances to the radio being much larger. Mirror glance durations varied with the purpose of the glance (simple detection vs. traffic checks) and glances durations to the radio varied with the task and the design of the radio. The radio glance distribution appears to be log normal. Interestingly, the differences in radio glance durations between studies were not significant. Table ** shows some of these details and percentiles of interest. Figure 7. Distribution of Glance Durations Source: Rockwell, 1988, p. 321. Figure 8. Distribution of Left Mirror Glance Durations Source: Rockwell, 1988, p. 321. Table 5. Glance Statistics (s) from 3 Studies Source: Rockwell, 1988, p. 320. In discussion these data, Rockwell states the following, years researchers studying “For car following and eye movements have found a 2 second rule, i.e., drivers are loath to go without roadway information for more than 2 seconds (and rightly so)” (Rockwell, 1988, p. 322) which has led to what is known as Two-Second Rule.” “Rockwell’s
For large commercial vehicles, the National Safety Council now recommends a minimum 6-s following distance under ideal conditions, in contrast to a 4-second rule used in the past (National Safety Council, 2012). These rules make good sense as guidance. However, it is unknown how effective they are in heavy traffic, where cut-ins occur when gaps between vehicles are large. Cut-ins are important because they can be crash inducing events. One use the 2 and 3 second rules as a basis for determining how long it is safe to look away from the road. Rockwell’s 2-Second Rule (1988) Rockwell (Rockwell, 1988) studied driver "in car" visual performance in 3 different experiments over a 6-year period involving 106 subjects participating in 200 test sessions. The sample was even divided between younger (less then 35 years old) and “mature” subjects (over age 45). Data were collected on expressways in light to moderate traffic. Many of the studies involved glances to “stereos,” AM/FM radios that were being sold in the 1980s, as well as glances to mirrors (Figures 7 and 8). Note that the mean duration for glances to the mirror was 1.1 s, vs. 1.44 s to the radio, with the standard deviation of glances to the radio being much larger. Mirror glance durations varied with the purpose of the glance (simple detection vs. traffic checks) and glances durations to the radio varied with the task and the design of the radio. The radio glance distribution appears to be log normal. Interestingly, the differences in radio glance durations between studies were not significant. Table ** shows some of these details and percentiles of interest. Figure 7. Distribution of Glance Durations Source: Rockwell, 1988, p. 321. Figure 8. Distribution of Left Mirror Glance Durations Source: Rockwell, 1988, p. 321. Table 5. Glance Statistics (s) from 3 Studies Source: Rockwell, 1988, p. 320. In discussion these data, Rockwell states the following, “For years researchers studying car following and eye movements have found a 2 second rule, i.e., drivers are loath to go without roadway information for more than 2 seconds (and rightly so)” (Rockwell, 1988, p. 322) which has led to what is known as “Rockwell’s Two-Second Rule.”
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