Since the 1930's, design speed has been a primary factor in selecting the vertical and horizontal alignments of U.S. roadways. Since about the same time, posted speed limits have been based on a statistical analysis of individual vehicular speeds observed on the roadway. The speed limit is set at the 85th percentile speed--that is, the speed below which 85 percent of motorists travel. Inherent to these procedures--procedures accepted and followed by the engineering profession--are two assumptions: (1) that motorists are able to decide an acceptable travel speed and (2) that the 85th percentile speed is reasonable to use as the posted speed limit.

Typically, highway design "incorporate[s] a significant factor of safety, that is, roadways are designed for a near worst-case condition." As a result, motorists often travel at speeds greater than the roadway's design speed, a practice generally considered safe. Nevertheless, concerns still exist where posted speeds exceed design speeds. In lawsuit situations, it might be difficult to convince juries that allowing motorists to travel faster than the design speed is prudent and/or safe.

In light of these concerns, the Texas Department of Transportation (TxDOT) conducted a nationwide study to identify problems with the relationships among design speeds, operating speeds, and posted speed limits and to solicit possible solutions to those problems. Results showed that the 85th percentile speed is the primary factor used to set a posted speed limit. Respondents also underscored the belief "that design speed should always exceed or equal expected operating speed." Kay Fitzpatrick, Brian Shamburger, and Dan Fambro documented the results of the surveys in "Design Speed, Operating Speed, and Posted Speed Survey" (Transportation Research Record 1523). METHODS

The study involved a design survey and a traffic operations survey. A survey was tailored for the 25 TxDOT districts, the 50 states, and the District of Columbia, and a combined survey was developed for 130 cities and counties. The design survey focused on whether expected operating speed is considered when designing a roadway and whether the design speed is reevaluated later. The traffic operations survey focused on liability concerns, procedures used to set posted speed limits, methods used when design speed is below operating speed, and procedures used to set advisory speeds on horizontal curves. Researchers also asked agency personnel if an engineering speed study precedes setting or changing speed limits.

Of 282 surveys mailed, 168 were returned, for a final return rate of 58 percent. The high response rate suggested significant interest in this topic, given that surveys of this type typically generate response rates between 20 and 50 percent.

RESULTS FOR DESIGN SURVEYS

Nineteen TxDOT districts, 38 state DOT engineers, and 51 city/county transportation engineers responded to the design survey. Ninety percent of the city/county transportation engineers indicated they use the design speed concept when designing roadways. The survey also asked whether anticipated operating speed or posted speed is primarily used in selecting a design speed. Collectively, more than half the TxDOT respondents, state design engineers, and city/county transportation engineers reported they consider the anticipated operating speed and posted speed limit in their design selection process. The majority of respondents concurred with a recommendation to consider anticipated operating speed when selecting design speed.

Nearly 70 percent of the TxDOT design engineers said their agencies had procedures to review or reevaluate design speed once the initial roadway design is completed. In contrast, less than 35 percent of out-of-state engineers and 33 percent of city/county transportation engineers indicated the use of such procedures. Participants were also asked if their selection of an appropriate design speed was influenced by an expected change in a facility over time, for example from a rural environment to a suburban one. More than half of the TxDOT design engineers and approximately half the out-of-state engineers responded affirmatively to this question. Many who responded negatively stated that "it is too difficult to justify lower design criteria knowing that traffic will initially operate at a higher operating speed."

When asked about litigation issues, only two out-of-state design engineers said their states had been involved in a lawsuit related to a posted speed limit that exceeded the design speed. One respondent replied that while this issue has been tangentially involved in some cases, "it has never been a substantive one."

Finally, participants were asked whether their agency has or had a roadway where the operating speed or the posted speed of a section exceeds(ed) the design speed and what actions were taken to lower the operating speed or the posted speed, or to warn drivers of the conditions. Approximately half the TxDOT respondents answered yes to the first part, but indicated no actions were taken unless there was an increase in crashes or a specifically unsafe condition present. More out-of-state design engineers responded affirmatively to both questions. Typically, the action taken involved installing advance warning signs.

RESULTS FOR TRAFFIC OPERATIONS SURVEYS

Twenty of the TxDOT districts, 41 state DOT traffic engineers, and 51 city/county transportation engineers responded to the traffic operations survey. To learn if design speed is significant in determining posted speed, participants were asked what factors they considered when determining the posted speed limit for an existing facility. Nearly all of the TxDOT traffic engineers felt the 85th percentile speed, recent crashes, and state-mandated maximum speed limits were more important than design speed. Only five said they considered the design speed when posting speed limits. Nearly all (95 percent) of the other traffic engineers said the 85th percentile speed was the key component when determining posted speed. Thirteen out-of-state traffic engineers indicated they considered design speed when determining posted speed; but again, the 85th percentile was the key factor used to determine the posted speed limit.

When asked how they set the appropriate posted speed limit for a new facility, almost half the TxDOT engineers suggested design speed was used as the initial speed limit; and then, after the facility was in operation, the 85th percentile was used to set the operating speed. State traffic engineers responded similarly. However, less than half the city/county engineers indicated a similar process. Instead many said that posted speed limits were initially based on adjacent roadway sections (for continuity) and modified later if needed. When asked if they perform an engineering speed study before setting or changing a speed limit, the response from the three groups was a nearly unanimous yes. The majority also said they used a ball-bank indicator (see Figure 1) to set advisory speeds on horizontal curves.

In its Traffic Control Devices Handbook, the Federal Highway Administration described the ball-bank indicator and its use:

FIGURE 1
Ball-bank indicator

Two state traffic engineers and one city/county transportation engineer indicated previous involvement in lawsuits. For the TxDOT districts, out-of-state engineers, and city/county engineers, installing advisory signs was the usual approach to lowering operating speeds or warning drivers of the conditions when the operating speed or posted speed of a roadway section exceeded the design speed.

CONCLUSIONS

More than 75 percent of the respondents agreed "that anticipated operating speed should be considered when selecting the design speed of a roadway." The authors commented, "As expected, the factor considered in almost every situation to set a speed limit on an existing road is the 85th percentile speed." The rationale behind the 85th percentile speed limit, however, is not well understood by the public or the police. While a ball-bank indicator was the primary method of setting advisory speeds on horizontal curves, a number of respondents voiced concern about its use. Although none offered suggestions or recommendations for an improved or alternate method, they felt a new procedure would be desirable.