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Federal Highway Administration Research and Technology
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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-05-042
Date: October 2005

Safety Effects of Differential Speed Limits

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Appendix A shows the initial data request letter.

Dear Mr. Baldwin,

As I mentioned on the phone, the Virginia Transportation Research Council is working with the Federal Highway Administration to evaluate the potential safety impacts of differential speed limits for cars and trucks on interstate facilities. I would like to request your assistance with obtaining crash and speed data (on interstate highways) that can help us with this study. While we already have some limited data for the sites shown at the bottom of this letter, I’d like to obtain some additional data pertaining to these and other sites.

I realize that obtaining data can be time consuming so I am certainly willing to do whatever is possible to make it easier for you to fulfill our request. If at all possible, we would like to obtain the following crash and speed data in an electronic format. The crash data and the speed data may come from the same sites or they may come from different sites, whichever is easier (but all should be on interstate highways, with the speed limit shown. At sites with differential speed limits please list the speed limits for passenger cars and trucks separately).

For each year from 1991 to 2000, we would like to obtain the following speed data elements at each site:

  1. Average speed.
  2. Individual vehicle speeds or speed bins (e.g., x vehicles between 51-55 mph, y vehicles 56- 60 mph, etc.).
  3. Individual truck speeds or average truck speeds (if available).
  4. Individual car speeds and average car speeds (if available).
  5. Critical geometric data such as:
    1. The number of lanes
    2. The number of interchanges (or the number of interchanges per mile)

The speed data sites that interest us are these plus any additional sites you recommend.

       Route 29NorthboundMilepost 29
       Route 35NorthboundMilepost 14

For each year from 1991 to 2000, I would also like to obtain the following crash data elements:

  1. Total number of crashes
    1. All crashes that do not involve trucks.
    2. All crashes that do involve trucks.
    3. All fatal crashes (regardless of truck involvement).
    4. All fatal crashes (of truck involvement).
    5. Total number of rear-end crashes that do involve trucks.
    6. Total number of rear-end crashes that do not involve trucks.

The crash sites should be the same from year to year, and can either be the sites shown above or be different from the speed sites. Each crash site should be a homogeneous section that can show some crashes (e.g., whether a crash site is 1 mi long or 10 mi long, it should be (a) big enough to obtain some crashes annually, yet (b) small enough such that speeds and geometric characteristics for the site are homogeneous). We would like, if possible, up to 10 crash sites altogether.

If possible, we would like the data to be in the following format. But we are also happy to have any format of data you sent to us.

RouteYearBeginning Mile PostEnding Mile PostAADTTotal CrashFatal CrashRear-EndTruck TotalTruck FatalTruck REMean Speed85th Speed

Finally, I would like to confirm that you have had since 1991 a uniform limit, that is, the same speed limit for cars and trucks.

Again, I sincerely appreciate your assistance. I would also be delighted to provide you with additional information about the purpose of this study, as well as any findings that result.

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