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Impact of Arterial Bus Rapid Transit on Traffic and Users

Video and statistical analyses showed that arterial bus rapid transit (ABRT) along Snelling Avenue in Minneapolis-St. Paul had no significant impact on traffic volume and wait times at intersections. Survey results demonstrated that users prefer the A Line over local bus service and consider it roughly equivalent to express bus, light rail and commuter rail service. Though ABRT has not converted automobile drivers to transit riders, users enjoy its easy payment format, cleanliness, route service and convenience. This study also provided recommendations for future ABRT line design considerations.

“Arterial bus rapid transit is perceived positively by users. It’s much like light rail and commuter rail—people think of it as equally useful as light rail.” —Alireza Khani, Assistant Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering

What Was the Need?

Bus rapid transit (BRT) entails dedicated lanes for buses and off-board payment for users who purchase fares before boarding the bus. In recent years, arterial BRT (ABRT) has developed as an alternative for metropolitan areas that lack roadway width for dedicated lanes. ABRT uses off-board payment but not dedicated lanes; instead, it uses existing roadway arterials and limited stops, offering a fast and efficient commute for users.

In 2016, the A Line opened on the Snelling Avenue corridor in Minneapolis-St. Paul, the area’s first ABRT line. It quickly gained popularity among transit customers as an alternative to local bus service, complementing the Twin Cities’ light rail system and commuter rail service from the suburbs.

Because the A Line operates within existing lanes of traffic and does not feature pullouts at its stops, it could slow corridor traffic when buses stop to load and unload. A preimplementation study of the corridor and A Line service suggested that traffic impacts would be minimal. The A Line’s actual impact on traffic, however, had not been determined, and user perceptions had not been assessed.

The A Line, Minnesota’s first ABRT line, has a positive reputation among riders and area residents,
and has had no negative impact on automotive traffic along Snelling Avenue, according to a recent
MnDOT study.
Passengers disembark from an A Line bus on Snelling Avenue.

What Was Our Goal?

MnDOT sought to examine the traffic impacts of the A Line in its first year of deployment, and to identify and quantify the A Line’s appeal to riders, including the service’s strengths and weaknesses, and how the transit experience of the A Line compares to local service. In addition, MnDOT needed to assess the characteristics of the service that could be used in new ABRT lines in the Twin Cities.

What Did We Do?

Researchers employed two strategies to evaluate A Line performance. First, the team conducted a traffic and transit capacity study. Investigators analyzed bus system data for ABRT and regular bus service capacity. In August 2017, researchers deployed four cameras each at two intersections: Snelling and University north of Interstate 94 (I-94), and Snelling and Dayton, south of the Interstate. Cameras collected video data for weeks before the 12-day Minnesota State Fair, which is held at the fairgrounds on Snelling Avenue, and additional video during the fair through its conclusion in September. Researchers analyzed recordings of four signal cycles before and after bus arrival at the intersections for traffic queues and volume.

Next, investigators studied the results of a 2016 Metro Transit survey of passengers on the A Line and four parallel standard bus lines. The study compared transit usage data from 2016 and 2017, before and after the A Line opened. The research team surveyed A Line passengers, station area residents, business workers and owners, automobile users, bicyclists and pedestrians. Team members also reviewed a recent study of Minneapolis-area real estate developers on transit facilities and options.

Traffic moves on northbound Snelling Avenue at Dayton in this image taken from the video analysis.
Researchers used video cameras at two key intersections along Snelling Avenue to evaluate the A Line’s impact on traffic.

What Did We Learn?

Video and data analyses revealed that the A Line increased overall transit capacity, and the time its buses spent not moving while passengers were loading and unloading during a green traffic signal had no significant impact on intersection queue length or traffic flow at the two intersections—during and outside State Fair dates. The A Line carries more riders than the local bus along the same route, and the greatest rider turnover occurs at the Snelling and University station, which connects with light rail service.

Surveys identified the five attributes most important to satisfactory transit service: easy fare payment format, hours of operation, complaint resolution, personal safety while riding and courteous transit drivers. A Line users were more satisfied with ABRT than with local bus service, and showed no significant difference in satisfaction with the A Line compared to express buses, light rail and commuter rail. For most individual service attributes such as payment procedures, travel time, shelter cleanliness, and route and bus signage, the A Line performed better than local buses, the same as light rail but not as well as commuter rail. Nonuser surveys indicated a positive perception of the ABRT, but mixed impact on pedestrian and bicycling activities and little impact on reducing preferences for using automobiles instead of transit.

What’s Next?

To improve A Line service, transit managers should focus on operating hours, the on-board safety of riders, reliability and total travel time. Researchers noted that rider satisfaction does not consider costs associated with improved service and recommended that future ABRT plans weigh improvements in the five key attributes of transit service against costs in planning new lines. The study findings and recommendations will be used in planning future ABRT lines.

“We will use this study to show MnDOT staff that arterial bus rapid transit should have minimal to no impact on existing traffic and signal operations.” —Carl Jensen, Transit Advantages Engineer, MnDOT Metro District

This Technical Summary pertains to Report 2018-35, “After Study of The Bus Rapid Transit A Line Impacts,” published December 2018. For more information, visit MnDOT’s Office of Research & Innovation project page.

Study Suggests 70 Percent RAP for Minnesota Gravel Road Surfaces

Researchers examined mixtures of recycled asphalt pavement (RAP) and aggregate for new gravel road surface layers in the lab and in the field. Although test results did not align perfectly, and field results were somewhat uneven, findings suggest that mixtures with 70 percent RAP content can reduce dust generation. After a year of service these roadways can match all-aggregate gravel road performance in terms of strength, but with a smoother ride.

What Was the Need?

Gravel roads offer a cost-effective option for road departments that wish to avoid the expense of asphalt and concrete roads in rural or low traffic areas. However, about an inch of gravel is lost from these roadways each year. Aggregate resources are diminishing, and gravel and crushed rock aggregate is growing increasingly expensive.

Gravel also generates dust that can reduce visibility, affect road performance and result in complaints from nearby homeowners. 

Recycled asphalt pavement (RAP) can be an effective component of new asphalt pavement mixtures. Many aggregate producers stockpile RAP that has been broken into the size of aggregate. But not all RAP works well mixed in asphalt, and some aggregate yards are too far from pavement projects to economically use RAP in pavement. 

Road agencies frequently use RAP in gravel roads. The asphalt content in RAP can bind with dust from crushed rock or gravel, helping manage fugitive dust. A recent study in Wyoming found that using RAP in new gravel surface applications at less than 50 percent of the aggregate resulted in good road performance and kept dust to a minimum. 

What Was Our Goal?

In light of the findings from the Wyoming study, researchers sought to determine the optimal level of RAP in an aggregate mixture for Minnesota gravel road surfaces. These new applications would offer good driving stability while also controlling fugitive dust. 

What Did We Do?

Research began with a review of the literature on RAP as an aggregate component of surface, base and subbase layers, as well as a survey of Minnesota counties on their experience with these mixtures. 

In the lab, the research team tested three RAP materials and virgin aggregate from two Minnesota locations in various RAP content levels for strength and compression. Investigators then compared the economic feasibility of 100 percent virgin aggregate use to 50 percent virgin and 50 percent RAP aggregate mixtures on a 1-mile aggregate road, including annual grading and eventual regraveling in the estimations.  

Research in the field focused primarily on six 1,000-foot gravel road test sections: four sections in Goodhue County using 15, 30, 45 and 60 percent RAP content, and two sections in Carlton County using 30 and 50 percent RAP. The studies entailed all-virgin aggregate control sections, and installations were made over roads with various subgrade soils that presented a variety of properties. Sites were tested for elasticity, bearing strength and fugitive dust generation. 

A secondary field study focused on RAP contents of 50, 70 and 80 percent in 3-inch surface courses for three test sections and one control section in Goodhue County. Sites were tested for elasticity, strength, dust generation, ride quality and surface aggregate looseness over time, and some lab tests were conducted.

“The 70 percent RAP mixture seemed to be about the best combination. We put RAP down in fall 2017, and by the next summer, it was working much like a regular gravel road.” —Charles Jahren, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering 

Mounds of RAP at a gravel pit in Carlton County offer road agencies an alternative to
natural gravel and crushed aggregate for gravel roads. But RAP has to be used in the
right proportion with gravel.
Mounds of RAP at a gravel pit in Carlton County offer road agencies an alternative to natural gravel and crushed aggregate for gravel roads. But RAP has to be used in the right proportion with gravel.

What Did We Learn?

Previous research indicated that RAP can help reduce fugitive dust, offers value as surface courses, and can reduce moisture susceptibility of gravel roads in cold or wet locations. 

Lab mixtures with 30 percent RAP consistently produced high compressive strength values, and higher RAP levels generally correlated inversely with bearing strength. Improvements in dust reduction were limited until RAP levels exceeded 50 percent. 

Economic analysis determined that a 50/50 percent mix of RAP and aggregate would cost 1.5 percent more than an all-virgin aggregate surface course in terms of construction and maintenance, but potential reductions in dust generation, surface aggregate loss and regraveling after three years of service may produce savings from RAP use. 

Results from field testing defied clear recommendations on optimal RAP content. Generally, higher RAP content offered greater elasticity and lower levels of loose aggregate initially, but these benefits fell to equal or below non-RAP levels after a year. Higher RAP correlated with reduced dust generation, but again fell over the first year of service. In secondary testing, initial dust generation was lower with the 50 percent mixture than the others, but after a year was lowest with the 70 percent mixture. 

Ultimately, researchers found that after a year, during which fugitive dust production was reduced, the performance of a 70 percent RAP content aggregate surface course was most like a virgin aggregate surface course and offered a smoother driving surface. 

What’s Next?

“These findings provide another tool in the toolbox. They will be most useful to engineers who haven’t used RAP in gravel roads and to county engineers who have a RAP resource.” —Joel Ulring, Pavement Engineer, MnDOT State Aid for Local Transportation

While this research did not develop a definitive recommendation for an optimal RAP content in surface courses for aggregate roads, it did produce useful data on performance. The study did encourage a general sense that 70 percent RAP content for surface courses of approximately 2 inches may be effective and warrants systematic study for a three-year period. 

A researcher scrapes a gravel road surface with a modified garden hoe to measure loose aggregate levels.
A researcher scrapes a gravel road surface with a modified garden hoe to measure loose aggregate levels.

This post pertains to Report 2019-11, “Optimal RAP Content for Minnesota Gravel Roads,” published March 2019. For more information, visit MnDOT’s Office of Research & Innovation project page.

Epoxy-Coated Rebar Bridge Decks Outperform Mixed Rebar Decks

Bridge decks reinforced with one layer of epoxy-coated rebar and a bottom layer of uncoated steel rebar show corrosion damage sooner than decks constructed with all epoxy-coated rebar. Inspection methods should be enhanced to add a rating for cracking density on the underside of bridge decks. Repairs to mixed rebar decks should be conducted once a key deck surface inspection element has received a condition rating of two and held that rating for seven years, which is sooner than the average repair time of 8.5 years.

What Was the Need?

In concrete bridge decks, steel reinforcing bars are necessary to add tensile strength and transfer loads to beams. Additionally, steel reinforcement in concrete bridge decks is designed to control cracking, which will extend the service life of the bridge.  

Steel also corrodes in salt environments, even when embedded in concrete. Water and road deicing chemicals can reach the steel and damage its strength and integrity. Between 1973 and 1990, MnDOT built approximately 660 bridges with more expensive, epoxy-coated rebar in the top layer of reinforcing matting and standard black rebar in the bottom layer. The coated top layer, only 3 inches below the deck surface, was expected to resist corrosion, and damage from salt and water would not reach the next layer of rebar, another 3 inches down in a 9-inch deck. 

In recent years, MnDOT has used another reinforcing strategy: mixing noncorrosive fibers into concrete mixes to help prevent or minimize cracking and resist corrosion. The older, mixed reinforcement bridges remain in service and few have been redecked. Performance of mixed reinforcement and fiber reinforcement in Minnesota bridge decks has not been compared to the performance of bridge decks constructed with only epoxy-coated rebar.

What Was Our Goal?

MnDOT sought to compare the performance of mixed rebar decks with all epoxy-coated rebar decks, and the performance of fiber-reinforced decks with no-fiber concrete decks. MnDOT also wanted to learn how to plan preventive maintenance efforts for mixed rebar decks.

What Did We Do?

Researchers reviewed reports from inspections, conducted every two years, for bridges with mixed reinforcement decks and decks with 3-inch strips of fiber reinforcement mixed into the concrete. They narrowed their review to bridge inspection data from 506 bridges with epoxy-coated rebar (including 35 control decks with all-epoxy rebar) built between 1973 and 1990, and 22 bridges with fiber-reinforced concrete and epoxy-coated rebar (including four controls with no rebar) built between 2012 and 2017. All of the bridges were inspected through 2017.  

Investigators then conducted site evaluations of 75 mixed rebar decks and 25 all-epoxy rebar decks, as well as 11 fiber-reinforced concrete decks with epoxy rebar and four without rebar. Site surveys focused on confirming the accuracy of recent inspection reports and recording signs of cracking, spalling and other deterioration conditions. 

Spalling has occurred because deicing chemicals and water have seeped into the concrete deck through cracks in the deck surface, causing uncoated steel reinforcement to corrode and concrete to spall.

What Did We Learn?

All-epoxy rebar decks outperformed mixed rebar decks, showing less cracking on the top and underside of the decks. Mixed rebar decks deteriorated at a quicker rate on bridges with steel beams than on bridges with prestressed concrete beams. Traffic levels and surface cracking did not appear to affect deterioration of decks in any group. 

“There’s not really a good visual inspection standard for quantifying cracking under the bridge deck, and that’s especially important for these types of bridges with mixed rebar. Using only epoxy-coated rebar in decks was a good idea.” —Ben Dymond, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering

Data sets were too small to draw any conclusions about possible differences in performance of fiber-reinforced decks compared to bridge decks that were not built with fibers. 

Individual bridge elements, such as bridge deck surfaces, have historically been rated from one (best condition) to five (worst condition). Mixed rebar decks earned element ratings of three to four more frequently than all-epoxy rebar decks, and visual surveys identified more deterioration on the underside of mixed rebar decks than all-epoxy rebar decks. 

The research team recommended amending bridge inspection procedures to add a new rating element for quantifying crack density on the underside of decks to anticipate and prevent spalling and delamination on the underside of mixed rebar decks. The team also recommended that once the deck condition element in mixed-bar decks holds a rating of two for seven years, more robust cracking sealing techniques should be considered to prevent it from reaching a rating of three. (For most bridges, that repair typically occurs after 8.5 years of service.)

Finally, the team recommended continued evaluation of fiber-reinforced decks as inspection data is collected over time. 

What’s Next?

“These findings may help us shift some priorities for repairing or replacing mixed rebar bridges. We will continue to advocate for the use of all epoxy-coated rebar wherever we anticipate high levels of chlorides.” —Nick Haltvick, North Region Bridge Construction Engineer, MnDOT Bridge Office

This research confirms that MnDOT’s current practice of using only epoxy-coated rebar in bridge decks remains a durable solution and offers the best long-term value in terms of repair needs. MnDOT will continue to evaluate fiber-reinforced concrete deck behavior and may adopt a rating method for identifying crack density on the underside of concrete decks. 

A crack underneath a bridge deck, revealing signs of leakage and corrosion of steel reinforcement bars.
Cracks on the undersides of a bridge deck can leak as chlorides corrode steel and seep through the crack. 

This Technical Summary pertains to Report 2019-09, “Deterioration of Mixed Rebar and Fiber-Reinforced Concrete Bridge Decks,”  published February 2019. Visit the MnDOT research project page for more information.