NTTA should set 75 mph speed limits

75 MPH speed limit signThe North Texas Tollway Authority (NTTA) can set 75 mph speed limits on much of its road system.

HB 1353, which was effective Sept. 1, 2011, basically says any road can have a 75 MPH speed limit (full text) if justified by TxDOT’s speed zoning procedure. Before then, 75 mph was only allowed in a few, politically-chosen  counties.

This speed zoning procedure generally requires the speed limit to be set at the 85th percentile speed of traffic.

What’s the 85th percentile speed? It’s the upper end of the “flow of traffic”.

Speed limits set at the 85th percentile speeds fully legalize this “flow of traffic”–the speeds of reasonable drivers–and only criminalizes demonstrably unsafe speeds. (more info)

What’s the 85th percentile speeds on NTTA roads?

Texas 121 TollOn the Sam Rayburn Tollway (TX 121), the average 85th percentile speed across the entire roadway is 74.1 mph. That rounds to 75 mph. Virtually all of the tolled part of TX 121 could be set to 75 mph with no hassle.

How about thePresident George Bush Turnpike logo President George Bush Turnpike? Not so fast. (Ha ha, get the pun?) 7 year old data has the entire road averaging a 71 mph 85th percentile speed. But wait, there’s more! 7 years ago, its speed limit was 60 mph! The speed limit is now 70, so the 85th percentile speeds have probably crept up. Plus there’s the new eastern extension, which may have changed traffic patterns. This entire road needs to be rechecked!
Dallas North Tollway logoThe last road is the Dallas North Tollway. It’s surprising:

  • Between I-35E and I-635 has a 73 mph 85th percentile speed! That rounds to 75 mph! It’s debatable if the sag curves under many of the bridges really need a 50 mph advisory speed. If that could be dispensed with, or raised to, say, 60 mph, then you could easily see a 70 or 75 mph limit on the old part of the DNT!
  • Between I-635 and the Collin County Line: 69 mph. This part could go to 70 mph.
  • Between the Collin County line and Sam Rayburn (TX-121): 73 mph, another 75 mph candidate!
  • Between 121 and the northern end: Surprisingly, only 69 mph! So the 70 mph limit there wouldn’t be changed. However, this measurement was taken when the speed limit was only 65. It’s 70 now, so the speed could have crept up a hair. This section needs to be rechecked.

If NTTA strictly followed the 85th percentile rule (which is only fair, since we motorists are expected to strictly obey traffic laws), and didn’t use arbitrary “engineering judgment” to force lower limits, we could see 75 mph limits on most of its road network.*

*A caveat: Because of a pointless, broken program called environmental speed limits, there may be red tape to raising the speed limit on DNT between Frankford Rd. and SRT (TX-121) and PGBT between I-35E and TX 78. None of the rest of the NTTA road system is subject to this program.

Here’s the raw data from NTTA: ORR-2-21-2012. This has the Excel files and strip maps. The Excel files have the speed checks. You’ll use the percentile function to get each location’s 85th percentile speed. The strip maps show where each speed check was run.

Comparing current data to last time period’s data in Postgres

I have a Postgres table where each row shows the number of traffic tickets written by each city per month.

I needed to show the percentage change between the current and prior month’s ticketwriting. For example, if a city wrote 1000 tickets this month and 500 last month, then I wanted a field in the row for this month to show a 200% increase over last month.

Here’s how I did it.

I first had to create two functions:

-- number of traffic tickets for
-- the given month and city
CREATE FUNCTION traffictickets(
                   cityName CHARACTER VARYING,
                   MONTH TIMESTAMP WITHOUT TIME zone
                ) RETURNS INTEGER
     AS 'select "TrafficTickets"
         FROM raw."OCA tickets per city"
         WHERE "Date" = $2 AND "City" = $1;'

Then I made a short SQL script that referenced this function:

UPDATE raw."OCA tickets per city"
     SET "Change" = "TrafficTickets"::REAL /
        (CASE WHEN traffictickets("City", "Date" - INTERVAL '1 month') = 0
         THEN -1
         ELSE traffictickets("City", "Date" - INTERVAL '1 month')

Viola, now each of my rows shows the percentage change between the current and prior month’s ticket counts! Except there’s a huge problem–the changes aren’t consistently meaningful.

If ticketwriting dropped month over month, the change field will be between 0 and 1: going from 100 tickets to 20 tickets the next month, the change is 0.2. But here’s the problem: if ticketwriting increased, the number will be between 1 and infinity. If ticketwritng increased from 20 to 100, the change is 5. Or if it changed from 2 to 150, the change is 75!

Yikes, I need some way of bringing these positive changes down to Earth.

I think I have a solution: show the change relative to the city’s average ticketwriting over all its years in my database.

I added a new function:

-- average number of traffic tickets written by
-- that city over all months in my table
CREATE FUNCTION averageticketsforcity(
                   cityName CHARACTER VARYING
                ) RETURNS REAL
     AS 'select avg("TrafficTickets"::real)::real
         FROM raw."OCA tickets per city"
         WHERE "City" = $1;'

Here’s the new version of the query:

UPDATE raw."OCA tickets per city"
     SET "Change" = ("TrafficTickets"::REAL -
      traffictickets("City", "Date" - INTERVAL '1 month')::REAL) /
      CASE WHEN averageticketsforcity("City") = 0
        THEN 1
        ELSE averageticketsforcity("City")

That seems to have fixed it. Now all my values appear to be equally meaningful to each other.

Design speed shouldn’t block higher speed limits

Thanks to a bill recently signed by the governor, the Texas Transportation Commission, which oversees TxDOT, may establish speed limits up to 85 mph on any state highway.

The bill uses the word “designed”. I’m afraid this may be misinterpreted to mean the civil engineering concept of “design speed”.

“Design speed” is not the maximum safe speed. It’s only a tool to guide road design. At best, it’s a conservative first guess of a speed limit; it’s often OK to set higher speed limits.

The TTC still has to change some rules before we can possibly see higher limits. They’ll probably adopt whatever TxDOT recommends. I sent the below letter to TxDOT to encourage them to not conflate the bill’s language with the civil engineering concept. I got a positive response, but the proof will come when the TTC adopts the speed zoning procedure revisions.

The letter:

HB 1201, which was just signed by the governor, allows the TTC to set a speed limit up to 85 mph on any road provided that “that part of the highway system is designed to accommodate travel at that established speed or a higher speed” and a standard engineering study was run.

This is a good thing. The old 70 mph limit was legislated in 1963. It’s now 48 years later, and 85 mph is perfectly safe on many roads with our drastically improved vehicle and road technology.

Here’s my concern: I hope the word “designed” in the revised statute will not be misinterpreted and become a roadblock to higher speed limits.

There is a concept of “design speed” in civil engineering. However, a design speed is a poor guide for a road’s true maximum safe speed for at least three reasons:

1. A road’s design speed that of its worst part. Suppose a 50 mph road has a 40 mph curve. By definition, the road’s design speed is only 40 mph. In the real world, the road should be signed at 50 mph, and yellow warning diamonds would be posted at the curve recommending 40 mph.

2. Design speeds assume characteristics of vehicles and road technology of the past. So a design speed established in 2011 will assume the inferior stopping distances, power, and safety of vehicles from many years ago. Even worse, most Texas rural roads were designed decades ago (e.g., back when cars had poor drum brakes, biased ply tires, weak horsepower, little safety equipment, dim headlights, no ABS or stability control, etc.). Design speeds established way back then will certainly understate what today’s on-road fleet can safely handle.

3. MOST IMPORTANTLY: Per the AASHTO, the design speed is merely “a selected speed used to determine the various geometric design features of a roadway”. Therefore, it is really only a theoretical/laboratory measurement. Its purpose is not to determine a speed limit.

The design speed must not be interpreted as a maximum possible safe speed. At best, it is only a conservative “first guess” of an appropriate speed limit; a road’s true safe speed may easily be higher.

To conclude, I ask that, as the TTC revises speed zoning regulations to accommodate HB 1201, that it not hamstring the speed zoning process with the civil engineering concept of “design speed”. Certainly in its use of “designed”, the legislature did not mean to invoke this specific concept. In doing so, Texas would misuse a theoretical, laboratory measurement whose purpose was never to be an absolute cap on speed limits.

(AASHTO is the American Association of State Highway and Transportation Officials.)

Unwarranted stop sign near White Rock Lake

[This is a letter I just sent to a Dallas civil engineer asking him to remove a recently-installed, unwarranted stop sign.]


Recently, an all-way stop sign was put up at West Lawther Rd. and White Rock Rd. on the west side of White Rock Lake. Through an open records request, I have received a copy of the warrants. I’ve attached those and your field observations to this email. [The warrants and field observations.]

I am concerned that this new stop sign does not meet the claimed warrants and therefore needlessly inconveniences road users, increases risk of undeserved traffic tickets, and created a hazardous situation.

The first warrant finds a need to control left hand conflicts. To prove this, I want to see obstructed sight lines, high traffic volumes, or some interaction of the two. In fact, none of this is a problem. I have driven through this intersection many times. Visibility is fine, and outside exceptional times (e.g., exceptionally-crowded events at White Rock Lake), the traffic volumes rarely require any wait for left turns. (Remember that traffic controls should be based on normal conditions, not outlier events for which the city already requires individualized, affirmative measures to control traffic.)

Here’s an aerial map from Google Maps:

This intersection only has two left turn conflicts: northbound White Rock Rd. turning left on westbound White Rock Rd. and southbound West Lawther Rd. turning left on southbound White Rock Rd.

There is no visibility problem for either left turn.

Left turn at northbound White Rock Rd. has no visibility problem either direction. Looking north while stopped at the stop line:

Looking west:

There is plenty of unobstructed sight to make a safe turn.

Southbound W. Lawther left turn to southbound White Rock Road also has acceptable geometrics to facilitate safe left turns.

This is the southbound W. Lawther view at the stop sign, looking south, and a little forward of the stop sign:

Traffic heading north, emerging from the park, will be going slowly. It will have just passed through a narrow railroad underpass, made a tight 90 degree turn, and will be slowing for a stop sign.

Same intersection, looking west:

Excellent visibility. Further note that eastbound cars may be approaching at less than the 30 mph speed limit. They will have just approached from an all-way stop at White Rock Rd. and Winsted Rd. Additionally, the traffic will be slowing for the sharp curve anyway; a 25 mph advisory speed is posted for eastbound traffic around the curve.

Here’s the irony: this all-way stop creates two new hazards:

  1. The stop sign encourages left-turning SB W. Lawther traffic to stop further back. Compare the following photo to the prior one of the intersection:

    Note that I was actually stopped a few feet pastthe stop sign; had I been stopped with the front of my car at the sign, as required by law, I would have even worse view of oncoming traffic. Additionally, eastbound traffic approaching on White Rock Rd. now has a worse view of southbound W. Lawther traffic. Below is a picture I took approaching on eastbound White Rock Rd.; the white car has already completely passed the stop sign before it became visible to me.

    Normally, this is roughly where rational motorists would stop before taking a left turn. The stop sign pushed this point further back.
  2. Left-turning southbound W. Lawther traffic used to only have to monitor one direction: eastbound White Rock Trail traffic. Thanks to the all way stop, southbound W. Lawther traffic has to negotiate both directions as all directions have equal right of way. This ambiguity causes confusion, which causes crashes.

Your own field observation confirms the absence of a traffic volume problem. In 30 minutes, you recorded 74 vehicles pass through the formerly uncontrolled part of the intersection (vehicles coming from EB White Rock Rd. or SB W. Lawther). That means the mere 9 cars approaching the intersection from the park have, on average, 24 seconds between potentially conflicting vehicles. Clearly, there is no problem here with traffic volumes.

The second warrant is whether there is a need to control vehicle/pedestrian conflicts near locations that generate high pedestrian volumes. Again, this warrant is unmet.

Given our relatively dry conditions over the past few months and no sidewalks, “high pedestrian volumes” will be readily apparent with grass trails. Indeed, you do see a pedestrian path on the south side of White Rock Rd. while west from the intersection:

But the rest of the intersection is notable for its lack of pedestrian activity. Here’s the view headed north from the intersection:

In other pictures in this document, you’ll find no additional evidence of pedestrian usage of the intersection.

A satellite photo makes it even clearer:

The only pedestrian traffic is heading west from the elevated trail (old railroad right of way), heading west to cross White Rock Rd. a few car lengths south of the intersection, then continuing west along the south side of White Rock Rd.

On top of that, there’s not even a valid reason for pedestrians to cross the intersection—the other side only has the fenced backs of houses, and safer crossing points exist within less than a minute of walking in either direction.

Given that the in-intersection pedestrian traffic is minimal, possibly nonexistent, and that there is no compelling reason for pedestrians to cross the road in the intersection, and safe alternative crossings are a brief walk away, if in-intersection pedestrian traffic is even considered a problem, it would be better to just ban pedestrian crossings at the intersection.

The third warrant asks whether there is a visual obstruction that will prevent safe vehicle movement unless conflicting directions are required to stop.

As clearly demonstrated above, the only visual obstructions were created by the all way stop.

To conclude, the all way stop sign at White Rock Rd. and W. Lawther Rd. meets no warrants, and it makes the intersection less safe. I respectfully ask that you revert the intersection to its prior state, where only northbound White Rock Road, emerging from the park, had a stop sign.

I look forward to hearing from you.


Aren Cambre

TCEQ begins to relax rules on DFW’s stupid environmental speed limits

The Texas Commission on Environmental Quality (TCEQ) is finally relaxing rules on the Dallas/Ft. Worth (DFW) area’s stupid environmental speed limits (ESLs).

Yes, I meant “stupid”.

Background: the TCEQ must improve air quality in parts of Texas that don’t comply with Clean Air Act standards. In 2001 and 2002, in 17 Texas counties around Houston and Dallas/Ft. Worth, all 70 and 65 mph speed limits were reduced by 5 mph. The TCEQ guessed this would cause a 5.5 mph speed reduction (they assumed average speeds are 10% over the limit) and a modest emissions reduction.

In fact, ESLs don’t clean the air. Arbitrary speed limit changes have little, if any, effect on actual speeds. Further, the TCEQ wildly overestimated ESLs’ emissions benefit. Compared to total emissions reduction needs, ESLs’ true air cleaning-contribution is between imaginary and a rounding error.

Here’s what arbitrarily low limits really do: turn safe motorists into lawbreakers, undermine respect for the law and police, and make roads less safe due to more speed variance.

The cost/benefit ratio is out of whack!

If that’s not bad enough, almost all ESL benefits come from heavy trucks, not passenger vehicles! This means the vast majority of hassled motorists are barely even contributing to this imaginary emissions benefit.

The Texas Legislature saw the stupidity of ESLs and prospectively banned them as of September 1, 2003. “Prospective”, however, means already-enacted ESLs can remain but new ones cannot be imposed. They had to do this because repealing all ESLs would cause Texas to lose federal highway dollars.

Now the TCEQ is the relaxing ESL rules for DFW: ESLs are now a “transportation control measure”. This means that, instead of explicitly requiring ESLs, DFW’s Clean Air Act implementation plan will only require their (imaginary) emissions reduction. Therefore, ESLs can be eliminated if their (imaginary) emissions reduction is made up by some new emissions reduction. Before this change, any ESL change required a thorough analysis and an EPA rubber stamp if Texas is to retain federal highway dollars.

What prompted this? Recently, the North Texas Tollway Authority (NTTA) raised many of its speed limits. A portion of its roadways were subject to ESLs–the Dallas North Tollway between Frankford Rd. and TX-121 and the parts of the George Bush Turnpike that were open before 9-1-2003. The NTTA justified disregarding ESLs because their (imaginary) emissions reductions were offset by the ongoing conversion to all-electronic tolling (elimination of toll booths).

Now what does this mean for other roads? Probably little. If the TCEQ and EPA wanted to practice intellectual integrity, they would have ditched ESLs long ago and done something else. But in the fantasy world of their bureaucrats, ESLs’ imaginary benefit is real, so they cannot be dropped outright without jeopardizing highway funds.

The only way I see additional DFW highway speed limits increasing is one of these scenarios:

  1. Some road improvement causes an emissions reduction and this improvement is A. is not already part of the DFW Clean Air Act implementation plan and B. affects a road that existed as of 2000, when the emissions plan went into effect.
  2. Overall DFW emissions reduction needs are lowered, which I think is unlikely given that we’re looking down the barrel of even more stringent standards.
  3. ESLs are technically defined as a 5 mph speed limit reduction from a 70 or 65 mph limit. Roads that have a 60 mph limit today, but used to have a 65 limit before 2001 (when the ESLs went into effect), could be rezoned for 70 mph and then have an 65 mph ESL.
  4. Roads never subjected to ESLs in the first place could get increases. These include any roads with a 60 mph or lower limit before 2001 or any road that didn’t exist before 2003.