10.2 Roadway Tradeoff Guidance and Cross-section Optimization
10.2.1 Overview
Four-lane undivided roadways are predominant in rural areas of Texas and have historically had reduced safety performance compared to other types of rural roadways such as four-lane divided and two-lane roadways. Due to the limited right of way (ROW) usually associated with four-lane undivided roadways, there is not always sufficient room to accommodate a four-lane divided roadway. Additionally, two-lane roadways can significantly improve safety performance by converting these to Super 2’s or other two lane derivatives.
Research has been performed to compare the safety and operational benefits of converting four-lane undivided highways and two-lane roadways into other cross section alternatives. The alternative cross sections were analyzed by slightly increasing the overall width depending on available ROW and clear zone needs.
These alternative cross sections include:
- Providing a four-lane undivided with a 4- ft median buffer;
- Providing a four-lane undivided with a two-way left turn lane (TWLTL);
- Converting a two-lane section into a Super 2; or
- Converting a two-lane section into a Super 2 with a two-way left turn lane (TWLTL).
Some of the key factors identified as being influential variables affecting rural highway safety are horizontal curvature, driveway density, shoulder width, and operating speed. These factors should all be considered when determining the appropriate alternative cross section.
Guidance on selecting cross-sections for rural highways is provided in
; in many cases, use of these proposed cross-sections in a 3R project can minimize the need for roadway widening by reallocating existing pavement width and restriping. The driveway activity index shown in
is defined as residential driveway equivalents per mile including driveways on both sides of the roadway. The equivalents for industrial and commercial driveways are as follows:
- A single industrial driveway is equivalent to three residential driveways
- A single commercial driveway is equivalent to twelve residential driveways
If it is anticipated that any of the inputs (e.g., AADT) will exceed certain thresholds during the Design Life of the roadway, consideration should be given to utilizing the preferred cross section for the anticipated higher threshold(s). Refer to
; and
for complete analysis and research for these types of cross-section optimizations.
AADT | Driveway Activity Index per Mile 1 | Truck Percentage | Preferred Cross-Section |
< 3,000 | Any | Any | Two-Lane Undivided/ Two Lanes with TWLTL |
3,000 – 15,000 | < 30 | Any | Super 2 |
> 30 | < 15% | Super 2 with TWLTL | |
15 – 25% | Super 2 with TWLTL | ||
> 25% | Four Lanes with TWLTL | ||
15,000 – 20,000 | < 30 | < 15% | Four Lanes with 4-ft Median Buffer 2 |
15 – 25% | Four Lanes with 4-ft Median Buffer 2 | ||
> 25% | Four Lanes with TWLTL | ||
> 30 | < 15% | Four Lanes with 4-ft Median Buffer 2 | |
15 – 25% | Four Lanes with TWLTL | ||
> 25% | Four Lanes with TWLTL | ||
> 20,000 | Any | Any | Four Lanes with TWLTL/ Four-Lane Divided |
Notes: | |||
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