Texas Statewide Planning Map zoomed in on Highway 79 east of IH-35 and west of Red Bud Lane.

Last night, the Round Rock city council approved extending lower speed limit zones on Highway 79 within the city limits. I wasn’t planning on writing about this. I found out about the proposed reduction of speed limits on Highway 79 late in the game, and almost never use Highway 79 anyway. The City Council’s concerns mostly (and entirely, during the second reading) were that the state was going to go ahead and reduce these speed limits anyway, regardless of what Round Rock did, so they might as well do the same, to be nice to the state.

However, I learned afterward that another concern is that there are an inordinate number of accidents along Highway 79 within Round Rock. That makes speed limits important, because, as the the Federal Highway Administration’s Methods and Practices for Setting Speed Limits puts it:

Setting a speed limit based on the 85th percentile speed was originally based on safety. Specifically, research at the time had shown that traveling at or around one standard deviation above the mean operating speed (which is approximately the 85th percentile speed) yields the lowest crash risk for drivers. Furthermore, crash risk increases rapidly for drivers traveling two standard deviations or more above or below the mean operating speed. Therefore, the 85^th percentile speed separates acceptable speed behavior from unsafe speed behavior that disproportionately contributes to crash risk.

The problem with reducing speed limits to address high accident rates along Highway 79 is that there is an alternative explanation for a higher accident rate that better fits the speed survey data performed by the Texas Department of Transportation: Round Rock’s speed limits along Highway 79 are already too low. They appear to be set significantly below the 85^th percentile.

There also appears to be some sort of jurisdictional blame-shifting going on. Round Rock says the state is asking them to reduce the speed limits, but when I talked to the Texas Department of Transportation, their representative said that the speed surveys were performed entirely at the request of the city.¹

Further, Texas has a State Planning Map that, rather than showing lower speed limits along Highway 79, shows higher speed limits than currently exist, and these higher speed limits seem to match what the 85^th percentile shows as the appropriate speed limits for the highway within Round Rock city limits.

If there is a higher accident rate along Highway 79 in Round Rock caused by dangerously low speed limits, extending the too-low speeds further is likely to make the problem worse.

Since the resolution reproduces only two of the speed surveys that were performed, I’m making the rest of them available here.

For IH35 to just east of Mays, the survey between Mays and 35 (PDF File, 58.7 KB) shows an 85th percentile of 37 westbound and 35 eastbound. Rounding down to 35 still means a quarter of westbound drivers become lawbreakers. However, the proposed change continues 232 feet east of Mays, for which no survey was performed. There was a survey at 1320 feet east of Mays (PDF File, 58.8 KB). At that point the 85th percentile increased to 46 in both directions.

However, the survey showing lower speeds between IH35 and Mays was performed on June 8, 2015, a Monday morning, at 9:00 AM. It’s strongly recommended not to perform surveys on Mondays or Fridays due to the potential for “exceptionally high traffic volume”. The higher speeds just east of Mays were found at 9:50 AM. The difference may simply be a matter of the time of day.

For just east of Mays to A.W. Grimes, the survey shows that except for lunch hour traffic east of Georgetown Street (PDF File, 58.7 KB), the 85th percentile ranged from 49 (PDF File, 59.3 KB) to 52 (PDF File, 60.8 KB). For example, of the east of Providence traffic (PDF File, 84.1 KB), 44% would become lawbreakers at the proposed 45 mph speed limit and 48% of west of A.W. Grimes traffic (PDF File, 60.8 KB) would become lawbreakers.

A better speed limit here seems to be 50 miles per hour or more.

For A.W. Grimes to just east of Red Bud Lane, the 85th percentile at Joe DiMaggio (PDF File, 58.6 KB) was 60 westbound and 62 eastbound. A 55 limit makes 62% of drivers into lawbreakers.

Similarly, the 85th percentile speed for east of Telander (PDF File, 59.2 KB) was 59 westbound and 67 eastbound. A 55 limit makes 50% of that traffic into lawbreakers.

The more appropriate speed limit for this section appears to be 65 or 70, based on eastbound traffic, or at least 60 based on westbound traffic.

Some of the surveys, as noted, do in fact support the lower speed limits. Most don’t, however, and those that do seem to coincide with bad times to perform a speed survey: Monday morning at 9 a.m., and Monday during lunch hour traffic, from 11:40 to 12:20.²

While Texas only requires that speed surveys be performed on “average weekdays during off-peak hours, under favorable weather conditions”, the U.S. Department of Transportation recommends that those average weekdays be Tuesdays, Wednesdays, or Thursdays, because Mondays and Fridays are often not “average weekdays”. As Iowa State’s Institute for Transportation puts it,

Traffic counts during a Monday morning or a Friday peak period may show exceptionally high volumes and are not normally used in the analysis; therefore, counts are usually conducted on a Tuesday, Wednesday, and Thursday.

There’s another reason for using the 85^th percentile to set speed limits, and that is to encourage respect for the law. Again from Methods and Practices for Setting Speed Limits:

The 85th percentile speed method is also attractive because it reflects the collective judgment of the vast majority of drivers as to a reasonable speed for given traffic and roadway conditions. This is aligned with the general policy sentiment that laws (i.e., speed limits) should not make people acting reasonably into law-breakers. Setting a speed limit even 5 mph (8 km/h) below the 85th percentile speed can make almost half the drivers illegal; setting a speed limit 5 mph (8 km/h) above the 85th percentile speed will likely make few additional drivers legal.

“It is important to note,” the manual says, “that setting speed limits lower than 85th percentile speed does not encourage compliance with the posted speed limit.” The effects of raising and lower the speed limit on the speed drivers go are, in fact, very small. The bigger effect is an increase in the accident rate, because traffic flow becomes disjointed.

Making otherwise reasonable drivers into lawbreakers encourages a lack of respect for the law. And in fact, because Texas has generally been very good about setting speed limits, at least on highways, I’ve occasionally gotten into trouble driving in other states when I drive at or very close to the speed limit. I end up driving dangerously slower than the rest of the traffic flow.³

In general, I’ve also been impressed, since moving here, by Round Rock’s willingness to revisit ideas that turn out to be bad ideas; last year they ended Round Rock’s speed camera program because the cameras were not meeting their professed goal of reducing accidents caused by running red lights. It may be that we’ll have to revisit this change, too.