-----Original Message-----
From: Nova SS [mailto:aren@cambre.biz]
Sent: Thursday, January 02, 2003 3:30 PM
To: 'cklaus@dfwinfo.com'
Cc: 'djodray@dfwinfo.com'; 'cedwards@dfwinfo.com'
Subject: Effectiveness of ESLs




I recently reviewed the TxDOT Dallas District’s surveys of actual roadway speeds one month before and 11 months after the 2001 implementation of environmental speed limits. These surveys were conducted at 18 sites across the Dallas half of the metroplex.


Here’s how the actual results compare to the NCTCOG’s and TCEQ’s expected results (as stated in TCEQ SIP documentation and on http://www.dfwinfo.com/trans/env_speed_limits/faq.html):





Average driver speed above speed limit


0.4% below SL before ESLs, 4.9% above afterwards

Actual speed reduction

5.5 MPH on ESL roads (70+10% - 65+10%) = (65+10% - 60+10%) = 5.5

1.6 MPH, and a few roadways even posted speed increases. Several of the roadways would have speed decreases without ESLs because of increased congestion due to suburban development.

Speed dispersion

Apparently not considered


Driver compliance with speed limit

No change

Greatly reduced

Enforceability of speed limit

No change

Far more difficult, especially considering that all Texas speed limits are prima facie.

Long-term compliance with speed limit

Apparently not considered

Decades of speed limit studies consistently suggest no long term compliance with arbitrarily low speed limits.

Emissions reduction

Contribute 1.5% of total emissions reduction goal

Contribute 0.4% of total emissions reduction goal, and probably closer to 0.1% after more accurate models run with EPA’s MOBILE6.


Decades of studies on speed limits and their effects on actual speeds repeatedly suggest that speed limits have no long-term relationship to actual speeds, so this 1.6 MPH speed drop is likely to be the best you’ll get. In fact, excessively restrictive traffic laws, including arbitrarily low speed limits like ESLs, only breed noncompliance with all traffic laws. This noncompliance encourages highly polluting and dangerous behaviors such as aggressive driving. Studies also suggest that nothing short of a never-ending, saturation-level law enforcement campaign could possibly bring about and sustain the needed compliance levels. Such a campaign would only cause antagonism, and it would undermine public support of the entire clean air initiative.


I strongly suggest that you review the Speed Limit portion of Synthesis of Safety Research Related to Speed and Speed Limits by the Federal Highway Administration. The FHWA is keenly interested in the problems caused by arbitrarily low speed limits. It is actively cooperating with interested jurisdictions to bring about higher, more rational speed limits. This sound movement is in direct conflict with the ESL program.


You can see the actual TxDOT Dallas District speed data at http://people.smu.edu/acambre/traffic2003/esl.htm.


Aren Cambre