Overview
The South Mountain Freeway portion of State Route 202 (State Route 202 SMF) opened in the Phoenix, Arizona metro area in December of 2019, completing a long-awaited by-pass for the downtown Phoenix portion of Interstate 10. Almost immediately the additional traffic from State Route 202 SMF headed northbound to westbound Interstate 10, merging onto Interstate 10, resulted in heavy backups through the interchange. To address these safety and congestion issues, the TSMO Division of the Arizona Department of Transportation (ADOT) evaluated the conditions and developed a plan to re-stripe the westbound (WB) corridor. ADOT added an additional through lane along WB Interstate 10 between 67th Avenue and Avondale Boulevard with striping changes only; no additional pavement widening was added. Congestion has been significantly reduced based on pace car driving and traffic camera observations. Average speeds have increased by approximately 33% and the travel times have been reduced by approximately 25%. This can be seen in the table as well as the heat map showing the congestion per time of day along the length of the corridor where almost all of heavy congestion (red) has been mitigated. Most importantly, crashes have also been reduced. Preliminary crash data is trending down, with fatal crashes reduced from approximately 0.1 per month to 0.0 per month. The total crashes have also decreased by approximately 6% and injury crashes (including: serious, minor, and possible injury) have reduced by over 30%. It was shown that in heavily congested corridors, reducing lane width and shoulder widths to gain an additional lane and eliminate a lane drop has not proven to result in a reduction of safety as was suggested by some in initial internal discussions.
In this case study you will learn:
- How congestion on a new Phoenix by-pass led to investigation of how merge back-ups could be reduced within a set pavement width footprint
- How ADOT added an additional lane using striping changes only with FHWA approval in order to add capacity
- How congestion was reduced through the striping plan, with a reduction of travel times of 25% and a total crash reduction of 6%
Background
The South Mountain Freeway portion of State Route 202 (State Route 202 SMF) opened in the Phoenix, Arizona metro area in December of 2019, completing a long awaited by-pass for the downtown Phoenix portion of Interstate 10. Almost immediately the additional traffic from State Route 202 SMF headed northbound to westbound Interstate 10, merging onto Interstate 10, resulted in heavy backups through the interchange. With over 80,000 vpd in the westbound direction alone, this segment of Interstate 10 accounts for significant time wasted in traffic along a section of highway that was already significantly congested. In addition, as with any congested area of freeway, the traffic crash frequency increased dramatically in the westbound direction.
To address these safety and congestion issues, the TSMO Division of the Arizona Department of Transportation (ADOT) evaluated the conditions and developed a plan to re-stripe the westbound (WB) corridor. ADOT added an additional through lane along WB Interstate 10 between 67th Avenue and Avondale Boulevard with striping changes only; no additional pavement widening was added.
TSMO Planning, Strategies and Deployment
The plan was to provide an additional continuous lane between the system interchange at the State Route 202 SMF and the system interchange at State Route 101 by narrowing the travel lanes to 11ft and reducing the shoulder widths as needed. The concept level plans for the additional through lane was designed by the Operational Traffic and Safety (OTAS) Group of the TSMO Division on an aerial map to speed up the process and initiate the required design exceptions. The concept design was then shared with the ADOT pavement preservation design team and incorporated into a previously scheduled pavement preservation project. The restripe/lane addition was implemented with the project on October 15, 2020, and improvements to both congestion and safety were seen immediately.
Communications Planning and Execution
The increase in the number of lanes was accomplished entirely within the existing pavement widths with no new construction. This work was coordinated with the planned restriping effort of the Interstate 10 which provided a new clean surface to tie into the newly constructed State Route 202 ramps. The previously identified design exceptions were requested based on non-typical in lane and shoulder widths related to design standards outlined in ADOT’s Roadway Design Guidelines and AASHTO’s “A Policy on Geometric Design of Highways and Streets (2018) and “Interstate Policy” (2016). For the proposed restriping plan, design exceptions were requested for the reduced travel lane width and reduced shoulder widths. This additional lane was expected to significantly increase capacity while reducing delay, weaving, congestion, and associated crashes. FHWA approved the design exception and the project proceeded as planned.
Outcome, Benefit and Learnings
Based on a very small sample size the project so far has been a great success. Congestion has been significantly reduced based on pace car driving and traffic camera observations. Average speeds have increased by approximately 33% and the travel times have been reduced by approximately 25%. This can be seen in the table as well as the heatmap showing the congestion per time of day along the length of the corridor where almost all of heavy congestion (red) has been mitigated. Most importantly, crashes have also been reduced. Preliminary crash data is trending down, with fatal crashes reduced from approximately 0.1 per month to 0.0 per month. The total crashes have also decreased by approximately 6% and injury crashes (including: serious, minor, and possible injury) have reduced by over 30%.
During this project the decision was also made to restripe the southbound State Route 101 to westbound Interstate 10 ramp to address that capacity and delay with similar speed and travel time benefits. This portion of the restriping was done a few weeks later and therefore crash data is not yet available. This ultimately reduced delay and crash costs to the public, reduces driver frustration and reduces automobile emissions.
One other important note is that in heavily congested corridors, reducing lane width and shoulder widths to gain an additional lane and eliminate a lane drop has not proven to result in a reduction of safety as was suggested by some in initial internal discussions.