October 24, 2002


Mr. Thomas H. Diggs, Chief, Air Planning Section

Environmental Protection Agency, Region 6

1445 Ross Avenue

Dallas, Texas 75202-2733


Mr. Diggs,


Following are my comments for the federal register on FRL-7383-6 concerning adjustments for the environmental speed limit program in the Houston-Galveston Area (HGA) Clean Air Act (CAA) attainment zone.




I urge a rejection of the currently proposed rule and future proposed rule that modifies environmental speed limit (ESLs) in any SIP unless the change is a full cessation of the ESL program. There is no sound basis to prove that tinkering with speed limits will cause the corresponding speed reductions necessary to reduce emissions. Without these emissions reductions, approval of and dependence on any kind of speed limit reduction scheme will jeopardize CAA attainment.


POINT 1: Speed limit reductions don’t simply mean corresponding speed or emissions reductions.

Decades of speed limit research and practical experience with the 1974 55 MPH speed limit show that speed limits have little effect on actual speeds for the following reasons:

  1. Drivers do not adjust their speeds unless they feel a direct impact to their pocketbooks or safety.3 For example, compliance with 1974 55 MPH speed limit was high in the beginning but quickly vanished once gas prices rationalized.1
  2. Speed limit adjustments have little short term impact on actual speeds. For example, New Jersey recently raised many of its highway speed limits from 55 MPH to 65 MPH. It noticed no speed change on freeways and only a modest change on turnpikes.2 Numerous substantial studies show little to no impact on actual speeds with speed limit changes.3, 4
  3. Speed limit adjustments have no long term effect on actual speeds. It is just common knowledge now that despite repeated threats of withholding of federal highway funds, compliance with the 55 speed limit was extremely low even in the ‘80s. In fact, by 1994, compliance with the 55 in New York state was only 4%.5
  4. Low speed limits discourage compliance with the law. This is actually a serious concern of the USDOT which is seeking to implement rational speed limit programs that will typically result in higher but more rational speed limits.6 ESLs directly conflict with the USDOT’s rational speed limits program.
  5. Low speed limits could directly cause more emissions from certain drivers. Aggressive driving produces many times the emissions of reasonable drivers, and aggressive driving in and of itself is a manifestation of a motorist’s lack of respect for the law. Any regulation that reduces respect for the law will increase aggressive driving and thereby increase emissions.


The TCEQ’s own documentation suggests that broad, unrealistic assumptions were made about driver behavior under the ESL program. For example, it assumed that the DFW-area 5 MPH speed limit reductions would cause roughly consistent 5 MPH reductions in actual speeds. It also assumed that the HGA-area 5 MPH – 15 MPH speed limit reductions will cause typical highway speeds to peak at 60-61 MPH. The Dallas District of the Texas Department of Transportation (TxDOT) conducted speed surveys at many representative locations on the Dallas side of the DFW area just before and almost a year after the 5 MPH speed limit reductions. I personally reviewed these surveys and found that actual speeds have declined on average by only about 2 MPH. (This is completely consistent with my own informal observations of the same roads.) Also, my September informal observations of six Houston-area highways strongly suggests that average speeds are well above the estimated 60-61 MPH on almost all roads despite the 55 MPH ESLs and aggressive police presence.


To summarize this point, the TCEQ has not demonstrated that reducing already low speed limits even further will have the desired effect on actual traffic speeds.


POINT 2: A 5 MPH speed limit reduction for 70 MPH and 65 MPH roads in the HGA aren’t enough of an emissions benefit to be worthwhile.

A recent reanalysis of HGA’s ESLs using MOBILE6 showed that the 55MPH ESL-induced emissions reductions are only about 1/3 of what was originally anticipated under MOBILE57. That means they will contribute about 0.5% towards the NOx emissions reduction goal if and only if drivers behave according to the TCEQ’s unrealistic expectations.


If speed limits are raised so that only the 70 MPH and 65 MPH roads bear 5 MPH speed reductions, then it’s very likely that there will be no demonstrable net emissions reductions. Again, remember that the Dallas-area’s enactment of the same program has resulted in a 2 MPH actual speed reduction. This reduction will certainly evaporate over time just like compliance with all low speed limits.


POINT 3: ESLs are not enforceable under existing Texas law.

Under Texas law, all speed limits are prima facie8. In other words, if a ticketed motorist successfully argues in a court that his speed was “reasonable and prudent,” the court must dismiss his ticket. The historical definition of “reasonable and prudent” refers only to the safe and the efficient flow of traffic.3 The state of the air does not affect safety or the flow of traffic, so theorized environmental benefits cannot play into the definition of “reasonable and prudent.” Therefore, any motorist ticketed under an ESL who was in compliance with the speed limit in effect before the ESLs must be found innocent if he uses this defense. An unsophisticated “boilerplate” defense is all that would be needed as suggested by Harris County District Attorney Chuck Rosenthal’s own statements.


To conclude, there is little sound basis that ESLs can achieve uniform and safe speed reductions and the expected emissions reductions.


Cited documents:

1: Speed Doesn't Kill: The Repeal of the 55-MPH Speed Limit, Cato Institute: http://www.cato.org/pubs/pas/pa346.pdf.

2: The 36 Month Study Report on the 65 MPH Speed Limit in New Jersey, State of New Jersey: http://www.state.nj.us/transportation/65mph081601.pdf

3: Managing Speed: Review of Current Practice for Setting and Enforcing Speed Limits, Transportation Research Board, National Research Council, 1998.

4: Synthesis of Safety Research Related to Speed and Speed Management, FHWA-RD-98-154, July 1998: http://www.fhwa.dot.gov/tfhrc/safety/pubs/speed/spdtoc.htm

5: Do Speed Limits Matter?, National Motorists Association: http://www.motorists.org/issues/speed/speed_limits_matter.html

6: USDOT Speed Management Team: http://www.nhtsa.dot.gov/people/injury/speedmgmt/index.html

7: 2007 On-Road Mobile Source Emissions Inventories For the Houston-Galveston Ozone Nonattainment Area, Texas Transportation Institute, March 2002: ftp://ftp.tnrcc.state.tx.us/pub/OEPAA/TAD/Modeling/HGAQSE/Contract_Reports/EI/Mobile6_EI_HG_2007.pdf

8: Texas Statutes, § 545.352. Prima Facie Speed Limits : http://www.capitol.state.tx.us/statutes/tr/tr0054500.html#tr056.545.352




Thanks for your acceptance of this comment.




Aren Cambre




email: aren@cambre.biz