DOTD Rejection1

Speed Zone

La 74 (I-10- La 73) Control 264 Section 03 Community of Dutchtown Ascension Parish District 61

Ref 03-99-301

Mr. Chuck LeMieux 13085 Depen Road Gonzales, LA 70737

Dear Mr. LeMieux:

This is in response to your November 12, 1999 letter to Mr. William Temple appealing the Department's denial of your request to lower the speed limit on La 74, and other attachments written by you to local officials on this same subject. You appear to be listing the following points in your writings as indications that a lower speed limit is justified:

1. Dutchtown School Road intersecting La 74, and school related traffic

2. Day Care center located nearby, and school children being shuttled between it and the school

3. Heavy plant traffic in the area which overlaps school traffic

4. Comparison with Cornerview Road and La 021

As a citizen of Louisiana you are certainly entitled to know the reasoning behind our answers, so I will provide this in the following points:

Speed Limit Policy: Speed Limit studies involve the collection of speed data, sampling 100 or more vehicles at one or more locations along the highway section. An engineer will make a visual inspection of the route, driving it in both directions, assessing the type of roadside development, traffic and pedestrian activity, type of traffic control at intersecting streets, accident data, road surface, shoulder conditions and alignment. Our design policy establishes the 85th percentile speed as the primary factor in determining speed limits. This is the speed that captures 85% of the motorists sampled (only 15% exceed this speed). Our policy states that speed limits should generally be set at the 85th percentile speed, which is a nationwide engineering practice, though not without exception.

AN EQUAL OPPORTUNITY EMPLOYER A DRUG FREE WORKPLACE

Mr Chuck LeMieux

January 6, 2000

Page 2

2. Schools and Day Care Centers: Day Care centers do not figure prominently as a consideration for setting speed limits because the children are car passengers that are not exposed to any unreasonable danger when attended by reasonably prudent drivers. It is obviously impossible to design highways to protect against all driver error. All of us who have driven for any length of time have made mistakes that could have killed us had we not been more fortunate. Schools are typically addressed by school speed zones. Our criteria for school speed zones is that the school must (1) be located adjacent to a state route (school property adjoins right-of-way), and (2) there must be an established school crossing (child pedestrians) on the state route. It is noted that the absence of the second criteria once again involves children only as car passengers.

3. Heavy Plant Traffic: Heavy peak traffic is not a reason for reducing a speed limit. In fact, heavy traffic tends to reduce speeds without the need for a speed limit reduction. It would be unrealistic to reduce a speed limit to attempt to force rush-hour traffic speeds during off-peak traffic periods.

4~ Comparison with La 621 & Cornerview Road: Below is a summary of speed data for La 621, La 3038 (Cornerview Road), and La 74, and the speed limit requests and disposition for each, of the 3 recent studies (all speed data in mph). Another important parameter in speed studies is the 10 mph pace speed, Speed limits are typically not set below the 85th percentile speed or the upper limit of the 10 mph pace speed, which can be thought of as a 10 mph sliding window that stops at the point where it captures the most vehicles:

621                 55                   36 -60                  45               51                   40- 49               45.9
3038               30                    25-45                  33               38                    27-36               33
3038               35                    25-43                  33               37                    29-38               33
74                   55                    32-64                  48               55                    43-52               47.8

Although there are certain site-specific conditions that may override the 85th percentile speed, it is a very reliable parameter in setting speed limits, because it represents the comfortable driving speed for the majority of motorists. Accordingly, speed limits set higher than the 85th percentile and upper limit of the 10 mph pace speed tend to be unsafe, and speed limits set below these parameters tend to have poor compliance and are not enforceable. I have noted on many occasions the general public's very strong perception that people tend to drive 5 mph over the speed limit, and if you lower the speed limit from 55 mph to 45 mph for example, people will lower their driving speed from 60 mph to 50 mph.

This simply has not been born out in our studies. The dynamic between posted speeds and driving practice can be somewhat complicated. What typically happens when speed limits are lowered to an unreasonably Iow speed, is that the more timid drivers and law abiding drivers tend to lower their speeds, and the 85th percentile speed changes very little (maybe only by I or 2 mph in many cases). The average speed lowers primarily because of the downward shift in the tow-end speeds. This can create an unsafe condition by increasing the speed range, which makes passing more tempting due to the slower speeds of the Iow-end motorists. This is the "Catch-22" of lowering speed limits, in addition to it being ineffective in reducing the speeds of most motorists. You will note that the 85th percentire speed measured for La 74 was 55 mph which corresponds to its current posting, and which is the appropriate posting for this section of roadway.

cc: Dr. Kam Movassaghi Mr. John Basilica Mr. Roddy Dillon Mr. William Temple Mr. Karl Finch Mr~ Peter Aliain