- SPECIAL REPORTS
- THE MAGAZINE
Anyone who lives or works in or near a major city in the United States knows that traffic problems are a part of downtown life. It is no different in Dallas, Texas, where "rush-hour" traffic often occurs for up to six hours a day during the week, moving at an agonizingly slow pace. A particular interchange near downtown Dallas that includes portions of freeways dubbed the "Canyon," "Mixmaster" and "Lower Stemmons" are critically congested and operate in stop-and-go traffic every business day. The Mixmaster itself has received the dubious distinction of being named one of the top 10 "Worst Commuting Bottlenecks' in the nation by the American Automobile Association. No significant improvements to roadway capacity have been made to these areas since they were constructed between 1958 and 1962. Minor improvements such as converting the inside shoulders to travel lanes have been made but are considered interim improvements until a long-term solution can be provided.
The Texas Department of Transportation (TxDOT) is evaluating the downtown freeways and interchanges to determine how best to alleviate the problem of road jams. From 1996 to 1998, the three corridors were studied as part of the Trinity Parkway Corridor Major Transportation Investment Study (MTIS), an effort that sought to develop a solution to the congestion in the Canyon, the depressed portion of IH-30 south of downtown; the IH-35E/IH-30 Mixmaster interchange on the western edge of downtown Dallas; and Lower Stemmons, the portion of IH-35E from the Mixmaster to SH-183.
The improvement endeavor has been dubbed "Project Pegasus" after the 30' x 50' flying red horse (the same logo still in use by ExxonMobil) that started rotating atop the Magnolia Petroleum Building in 1934. The pegasus icon was a landmark of downtown Dallas for many years, but had fallen into disrepair. Through recent community efforts, however, the Pegasus was restored to continue its aerial reign as a shining beacon of the Dallas skyline. Similarly, the aging freeways that bypass downtown Dallas need to be rebuilt and restored to operate more smoothly and efficiently. Symbolizing this redefining opportunity, TxDOT launched "Project Pegasus... Transforming our Downtown Freeways."
Data Collection DifficultiesTxDOT contracted with Carter & Burgess Inc. of Fort Worth, Texas, a full-service civil engineering and consulting firm established in 1939, to provide survey control for a schematic design for the reconstruction of the Project Pegasus area. Highly detailed and accurate mapping, along with horizontal and vertical clearance data, was critical to developing the improved design. From the onset, it was recognized that the design corridor would present some challenges for data collection. In addition to being highly congested, it has limited available rights of way and contains numerous existing structures-some historic-that need to be preserved throughout the design.
The elements that needed surveying included interchanges, railroad crossings, flyovers (a bridge that connects two intersecting highways by going over the mainlanes), on-ramps, off-ramps, building clearance and power line clearances. Without lane drops that would make a bad traffic situation even worse, conventional surveying methods were essentially ruled out, as they would have been dangerous or difficult, and in some cases impossible. With this combination of obstacles present, it was clear that traditional methods alone would not be the most efficient method of obtaining the data. TxDOT needed a better solution.
Lenard V. Wall, RPLS, the Dallas district survey manager for TxDOT had some experience with laser scanning through a number of previous small contracts with the Laser Geomatics division, a part of Bohannan Huston Inc. (Dallas, Texas). Each of these smaller contracts provided TxDOT with valuable survey information that, in turn, established confidence in the technology. Wall believed that using laser scanning technology on Project Pegasus would be time-efficient, safe and allow us access to areas hard to reach using traditional methods. So, Wall charged our Laser Geomatics division with providing this data; we used a Cyrax 2500 ground-based LiDAR unit (now rebranded the Leica HDS 2500) from Cyra Technologies, a division of Leica Geosystems (San Ramon, Calif.) for several locations along the proposed route.
Scanning by SectionsWhen the Houston Street Viaduct opened in 1912, Dallas-ites laid claim to the longest concrete structure in the world. This open spandrel arch bridge (a bridge with open spaces between the deck and the arch members allowing visibility through the bridge) spans the Mixmaster area of Project Pegasus and the Trinity River, and carries traffic southbound from downtown Dallas to the city of Oak Cliff, Texas.
The Houston Street Viaduct
As we planned the data collection for this structure, we wanted to be sure to capture the detail of the broad arches, ornate railings and lamp posts for preservation and archival purposes, while at the same time collecting the critical engineering data needed for the upcoming Project Pegasus. At 5,106 ft long, it required 10 days of scanning, more than 150 scans, and hundreds of control points to capture the entire bridge and road surface. Our strategy was to scan the west side of the bridge north to south, the east side of the bridge south to north, and finally the deck from north to south.
On a structure of this size and length, with so many scans and control points, it is a challenge to keep track of the scans, the control points and their relation to one another. Two steps in our field workflow saved us countless hours in the office. By keeping an ongoing field log of the scans and their associated control points, and by taking digital photographs of each scan setup, we provided our office personnel with descriptive information that helped them to sort through field errors (misnumbered control points, etc.) as well as to understand features and geometry that they could not have seen for themselves in the office. This proved to be a valuable method of operation throughout the project.
Deliverables for this portion of the scan project included vector linework in MicroStation format (Bentley Sytsems Inc., Exton, Pa.), that defined the limits of each arch as well as footprints of the abutments and each pier. In addition, a GEOPAK (by Bentley Systems) digital terrain model was created for the deck and for underneath the bridge excluding those areas obscured by vegetation. While the deliverables to the client totaled less than 24 Mb, our database actually holds more than 800 Mb-much more data than required, data we can provide our client in the future if requested and that won't require us to revisit the structure.
Woodall Rodgers-I-35E InterchangeSeveral more tests were evident for the Woodall Rodgers/I-35E interchange. First, we needed to collect a large amount of low chord and pier data on a very high, arching flyover while collecting the surface information below. Second, this area of the interchange is expansive, requiring extensive setups and mobilization times associated with each new setup. Finally, Dallas Area Rapid Transit (DART) railroad tracks pass within this interchange, effectively splitting the collection area in half.
With careful planning, we collected data of the flyover, the Woodall Rodgers eastbound and westbound bridge, and the eastbound Woodall Rodgers to northbound I-35E ramp area in a mere four days. Using our Leica HDS 2500 laser scanner, we collected 59 scans-24 million survey grade points to define the required geometry for this interchange.
IH-30/IH-45 InterchangeThe IH-30/IH-45 interchange is a typical high traffic volume, multilevel urban interchange. After planning our data collection strategy and setting control points, which took about half a day, we scanned the entire interchange in one and a half days with no lane closures, no traffic delays, and without compromising the safety of our two-man laser scanning crew. Due to the stop-and-go traffic on I-30 during the morning and afternoon peak traffic times, our strategy included avoiding the lower part of the interchange except between 10:00 a.m. and 3:00 p.m.
The deliverables for this portion of the project consisted of vector linework in MicroStation format that defined the pier, abutment and low chord locations for each level of the interchange. In addition, a GEOPAK digital terrain model was created of the lowest level (IH-30) to better define the area of rehabilitation. To define the geometry of this interchange, we collected 22 scans, which equated to approximately 10 million survey grade points.
I-35E Utility CrossingAs with the entire project, heavy traffic volumes made acquisition of the data required to define the geometry of this utility crossing an obstacle. Once again, we strategically planned our scan locations to facilitate data acquisition. By doing so, we reduced our field time to approximately one and a half days. As with all of our scanning, at no time were any of our personnel in traffic lanes, and collecting the data required no lane closures.
Railroad CrossingsConsistent with most downtown highway projects, there were multiple locations where railroad crossings infringed on the project area. The DART light rail commuter system, the Union Pacific Railroad and the TRE Railroad all use the railroad crossings in the Project Pegasus area. Access to railroad rights of way are difficult to obtain and accommodations must be made for the movement of the vehicles. Remote data collection was mandatory for us to collect the required survey information. We collected survey data on a rail split, provided cross sections of a DART rail line parallel to I-35E, and top of rail data on two crossing structures without infringing on railroad rights of way. And since all of these bridges in some way impacted the future design of the roadway, Laser Geomatics also collected data on the underside of the bridge structures at two of the railroad crossings.
Building ClearanceAt first inspection, scanning building clearances seemed to be one of the least important areas for Project Pegasus. However, providing clearance information between the existing RL Thornton Bridge and the nearby billboard was essential to the engineers so that the removal of the billboard did not become an issue. Since the city of Dallas no longer allows for new billboards to be built, the owners of the existing billboard would be entitled to future earnings on the structure if it were removed.
While it took only half a day to define this geometry, the vector linework of the billboard and bridge that defined the spatial relation between the two provided the engineers with the needed data to make the correct decision regarding roadway alignment in this area-very valuable data.
Quality Results on DeliveryAn essential part of this project was conventional survey control. Since much of the scanning was linear in nature and the deliverables needed to be in the project coordinate system, we chose to register the scans to the project coordinate system rather than to themselves and then into the project coordinate system. This method requires tight survey control to maintain accuracy. Carter & Burgess provided the required conventional surveying and did an outstanding job. The firm had a survey crew work along with our scanning crew throughout the entire project. As we determined where our control points needed to be they were able to acquire the survey data for those points using conventional GPS and reflectorless total station instruments. That data was supplied to our post-processing group at the end of each phase via E-mail on an Excel spreadsheet.
The required deliverables for Project Pegasus were defined to accentuate the existing survey data that Carter & Burgess had available and fill gaps in the data necessary to complete the design. MicroStation design files were created that included vector linework that defined the low chord of all crossing structures, piers, columns and other bridge structures that were hidden for aerial survey. These MicroStation graphics were generated from the acquired point clouds, mapped to the local coordinate system with accurate vertical dimensions and supplied in TxDOT graphic standards. GEOPAK digital terrain models were created for the surfaces under the bridges to define the road surfaces. These deliverables were referenced to the existing survey data to provide a comprehensive, three-dimensional digital environment to produce the proposed design. While not all of the data we collected was used to create the DTM surfaces and vector linework, having this data archived allows us to respond to additional data requests quickly and efficiently.
The Project Pegasus laser scanning project required 364 separate scans, which were acquired in a period of six weeks. The survey data was acquired without closing any lanes of traffic, impeding the traveling public or placing employees in dangerous situations. More than 3.6 gigabytes of digital spatial data was collected and stored in individual databases. Laser Geomatics used this data to provide 12 MicroStation design files and 16 GEOPAK digital terrain models. The total size of the deliverables was a little more than 124 megabytes. Office personnel post-processed the data in an eight-week period and final deliverables were submitted to TxDOT 11 weeks after Laser Geomatics was given a notice to proceed. In many cases, the data delivered from the ground-based LiDAR would have been unavailable from traditional surveying methods and added a level of confidence in the proposed design. This confidence of the existing conditions allowed engineers to focus on design issues rather than identification of potential problems. The scanning data provided for Project Pegasus demonstrates the valuable addition that ground-based LiDAR can contribute in a comprehensive surveying situation.
Bryan Copeland, PE, was the design engineer for Carter & Burgess, and the main end user of our data. "The data worked very well for challenging existing conditions, especially for the top rail on two railroad structures, the Houston Street viaduct arches and the transmission line crossing," he says. "Since the data was so easy to work with and required no additional manipulation, it provided a confidence in our design that was not available before. Having the LiDAR data provided a tremendous time savings in the design process overall."
The data collected on this project will ultimately help fulfill the final MTIS recommendations, including more than $1 billion in multi-modal transportation improvements to the Mixmaster interchange and IH-30 and IH-35E freeways, extending the Woodall Rodgers freeway, high occupancy vehicle (HOV) lanes, a new location parkway/reliever route, light rail, bicycle and pedestrian improvements, Intelligent Transportation Systems and employer trip reduction programs. These implementations will help relieve some of the painful congestion that has plagued commuters in the downtown Dallas area, and perhaps help the Mixmaster area to lose the unwanted honor of being in the top 10 worst bottleneck areas. This is good news for those who might want to drive by that shiny red pegasus.
Bradley E. Adams, PE, is a senior vice president for Bohannan Huston Inc. (BHI) with 18 years' experience across many disciplines of the engineering spectrum. Adams manages the Laser Geomatics division of BHI that provides ground-based LIDAR. Prior to joining BHI, Adams was involved in many major projects in the Dallas-Fort Worth metroplex, including the design of the Dallas Area Rapid Transit (DART) light rail system, multiple projects at Dallas-Ft. Worth International Airport, numerous Texas Department of Transportation projects and the Super Conducting Super-Collider. Adams also served as a technical manager for Intergraph Corporation, Huntsville, Ala. James V. Flint, PE, is the systems manager for the Laser Geomatics division of BHI. Flint gained most of his 12 years of engineering experience in the structural engineering division of BHI as a project manager and bridge design engineer, and also has experience in water resources, community development, construction inspection and highway design. Flint is an FHWA certified bridge inspector with seven years of experience.