SITE CHARACTERISTICS

The number of experimental sites on nonlimited access highways and their section length, stratified by area type and level of speed limit change, is shown in table 1. Speed limits were lowered as much as 20 mi/h (32 km/h) at some sites and raised as much as 15 mi/h (24 km/h) at other locations. The study encompassed 59 sites where speed limits were lowered and 41 sites where speed limits were raised. Of the 100 sites, 6 roadways were multilane covering 14 mi (23 m). The remainder of the sections were two-lane highways. While 63 sites were classified as rural (area population less than 5,000), 24 of the rural sites were located on primary highways that passed through small towns and unincorporated areas. The volumes at the sites ranged from 300 to 17,000 vehicles/d. The average volume for all sites was 4,500 vehicles/d. A summary of the characteristics of the individual sites is given in appendix A.

As previously stated, the study sites were drawn from locations where States and localities made speed limit changes as part of their routine activities. Apart from the small town sites, the vast majority of the sites were located in urban and suburban fringe areas where changing land use patterns and traffic demands dictated a need for a speed limit revision. While a 15-mi/h (24-km/h) increase or a 20-mi/h (32-km/h) decrease in the speed limit appears to be an extreme variation, examination of the operating speeds at these sites revealed that the new posted limits were less than the 85th percentile speed. Consequently, these limits did not encourage motorists to drive at speeds higher than felt to be safe and reasonable by the majority of road users.

Although the 85th percentile speed was one factor considered by the States and localities in making the speed limit changes, other factors were used in the decision-making process. Based on speed data collected at sites where speed limits were raised, on average, the before speed limit was posted at the 20th percentile speed, ranging from a site posted at the 1st percentile speed to a site posted at the 70th percentile speed. After the speed limits were raised, on average, the speed limit posted was at the 58th percentile speed, ranging from a site posted at the 10th percentile speed to a site posted at the 97th percentile speed. Before the speed limits were lowered, the typical site was posted at the 78th percentile speed, ranging from a site posted at the 20th percentile speed to a site posted at the 99th percentile speed. After the speed limits were lowered, on average, the speed limit was posted at the 33rd percentile speed, ranging from a site posted at the 1st percentile speed to a site posted at the 75th percentile speed.

Shown in table 2 is the distribution of the study sites by percentile speed posted before and after the limits were changed. As illustrated in table 2, the speed limits at 29 percent of the sites where speed limits were raised were set between the 70th and 90th percentile speed. On the other hand, only 5 percent of the sites where speed limits were lowered had limits set between the 70th and 90th percentile speed. These results suggest that current nationwide practice is to set speed limits well below the upper range of the minimum risk band, i.e., below the 70th to 90th percentile speed.


  Table 1.  Experimental sites by area type and level of speed limit change.

-------------------------------------------------------------------------------
                              Speed Limit Change, mi/h                |
   Area*      ----------Lower----------         ---------Raise------  |  Total 
               -20    -15    -10     -5         +5      +10     +15   |
-------------------------------------------------------------------------------
Rural                                                                 | 
  Sites       1      8     22      7          16        7       2     |   63
  Miles       1.03  18.45  29.04  12.52       64.24     9.86    1.12  |  136.26
                                                                      |
Small Urban                                                           |
  Sites       1       -    11      5           3        2        -    |   22
  Miles       0.80    -     9.19   5.49        2.21     2.03     -    |   19.72
                                                                      |
Urban                                                                 |
  Sites               -     2      2           7        3       1     |   15
  Miles               -     1.25   1.88        8.15     3.01    1.74  |   16.03
-------------------------------------------------------------------------------
Total                                                                 |
  Sites       2      8     35     14          26       12       3     |  100
  Miles       1.83  18.45  39.48  19.89       74.60    14.90    2.86  |  172.01
-------------------------------------------------------------------------------
1 mi/h = 1.61 km/h
  1 mi = 1.61 km
                  *Note:  Rural = area population <5,000
                    Small urban = area population 5,000 to 49,999
                          Urban = area population >50,000


  Table 2.  Distribution of speed limits at sites by percentile speeds.

-------------------------------------------------------------------------------
           Before                      After Lowering Limit            |       
     Percentile Speed Posted         Percentile Speed Posted           | Total 
                           <30    30-50   50-70   70-85   85-9O   >9O  | Sites 
-------------------------------------------------------------------------------
                                             Number of Sites           |
               <30           3      -       -       -       -      -   |   3
                                                                       |
             30-70          11      1       -       -       -      -   |  12
                                                                       |
               >70          10     20       9       3       -      -   |  42
===============================================================================
           Before                      After Raising Limit             |       
     Percentile Speed Posted         Percentile Speed Posted           | Total 
                           <30    30-50   50-70   70-85   85-9O   >9O  | Sites 
-------------------------------------------------------------------------------
                                             Number of Sites           |
               <30           6      7      11       4       3      1   |  32
                                                                       |
             30-70           -      -       3       4       1      -   |   8
                                                                       |
               >70           -      -       -       -       -      1   |   1
-------------------------------------------------------------------------------

After the speed limit changes were made at the experimental sites, on average, the speed limit was posted at the 43rd percentile speed. By way of comparison, the speed limit posted at the comparison sites, where no speed limit changes were made, was, on average, the 45th percentile speed. These data suggest that the participating States typically post speed limits below the 50th percentile speed.

Shown in figure 5 is the range in 85th percentile speeds by posted speed limit after the limits were changed on the experimental sections. On average, the speed limits were posted 5 to 16 mi/h (8 to 26 km/h) below the 85th percentile speed. The largest difference occurred at a site with a 47 mi/h (76 km/h) 85th percentile speed and a posted limit of 25 mi/h (40 km/h). At the other extreme, one site had an 85th percentile speed of 41 mi/h (66 km/h) and a posted limit of 45 mi/h (72 km/h).

The range in average speeds by posted limit is presented in figure 6. The data illustrate that the posted limits were set slightly below the average speed of traffic for the majority of sites.

The range in 85th percentile speeds by posted speed limit for the comparison sites is shown in figure 7. The data indicate that the speed limits were posted 4 to 12 mi/h (6 to 19 km/h) below the 85th percentile speed. These findings are similar to those at the experimental sites, i.e., on average, the speed limits were posted below the 50th percentile speed or average speed of traffic.

(Skip Figures)

[Figure 5]

[Figure 6]

[Figure 7]

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