A summary of the traffic study submittals for the Delaune Estates development located on LA 73 is as follows:
The study scope for the project was established at the Ascension Parish Pre-Application Meeting. The size of the proposed subdivision and the existing conditions of the LA 73 corridor warranted a Threshold 2 Traffic Impact Study. A Threshold 2 study is required when a development is expected to generate between 41 and 400 trips within a peak hour. The Parish also requested that a crash review would be conducted. the study intersections included LA 73 at White Road, LA 73 at Post Office Road and LA 73 at LA 621. QES requested that the Parish provide projected traffic information for currently approved planned developments along the LA 73 corridor including the Baton Rouge Medical Hospital and Bluff Elementary School. The Parish provided the traffic study for the hospital but could not provide a traffic study for the school. However, a site plan was provided to QES in order to project the number of trips associated with the school based on the total building square footage. The trips assigned to the LA 73 corridor associated with the school and the hospital were referred to as “No-Build” Trips. These trips added to the existing counts produced the “No-Build Scenario”. The original traffic study was completed in December 2019.
According to the Ascension Parish Traffic Impact Policy, the objective of a traffic impact study as it relates to the capacity analysis is to maintain or improve the existing LOS (Level of Service). The policy states “An overall LOS ‘D’ shall be considered acceptable. Where LOS ‘D’ is not existing or the existing LOS cannot be achieved with improvements/mitigation, a description of impacts, constraints, mitigation measures, and results shall be provided.”
The Traffic Impact Policy refers to the overall Level of Service D. As a matter of common traffic engineering practice, based on the Highway Capacity Manual, the Overall LOS for a signalized intersection is defined in terms of a weighted average control delay for the entire intersection. Control delay quantifies the increase in travel time that a vehicle experiences due to the traffic signal control. Control delay is a complex measure based on many variables, including signal phasing and coordination, signal cycle length, and traffic volumes with respect to intersection capacity and resulting queues. The original traffic study, dated December 2019, met the criteria of approval as all study intersections were found to achieve an overall level of service “D” or better with both the subdivision and the other planned developments in place. Table 1: LA 73 at White Road/Duplessis Road Analyses (TIS December 2019)
In the original traffic study, the intersection of LA 73 at LA 621 was observed to be an overburdened intersection during the morning peak hour. The existing overall LOS of the intersection was determined to be a LOS “F” due to spillback from the I-10 heading west. The interstate was undergoing construction at the time to widen the I-10 segment between LA 73 and Highland Road from two (2)-lanes to three (3)-lanes in each direction. The December 2019 study analyzed the intersection with the assumption that the I-10 widening from two (2)-lanes to three (3)-lanes would eliminate the spillback from the I-10 ramp into the intersection of LA 73 at LA 621. This was an engineering assumption as observations revealed the cause for the congestion along LA 73 was due to the lack in capacity within the I-10 segment. The assumption was utilized as the scope of work agreed upon did not include the collection of data on the interstate or the interstate ramps. As seen in Table 3, once the I-10 improvements were in place, it was projected that the intersection of LA 73 at LA 621 would operate at Overall LOS C and D during the AM and PM peak hours, respectively. Table 1 and Table 2 show that the intersections of LA 73 at White Road and LA 73 at Post Office Braud (sic) would operate at Overall LOS C and B during the AM and PM peak hours, respectively.
Two supplements to the traffic study were performed showing analysis results in response to ERA comments. Tables 4-6 show the results from the “Additional Analysis #2” which was submitted on March 2, 2020. These tables were used in the ERA review and considered by the Ascension Parish Planning Commission at the public meeting on March 11, 2020. As with Tables 1-3, the results show no Overall LOS below D at any of the study intersections with the assumption that the I-10 widening would relieve the LOS issues at the intersection of LA 73 and LA 621.
Tables 4-6 were also considered by the Ascension Parish Board of Appeals on July 27, 2020. A main point of concern was that the analyses performed for the development relied too heavily on assumptions. The first assumption was that the amount of traffic from the new school and medical facility was projected, not based on actual conditions because the developments were not open yet. The second assumption was that the I-10 widening would alleviate the spillback of vehicles from I-10 to LA 73 which was causing issues at the intersection of LA 73 and LA 621.
A revised traffic study was performed by QES in response to these concerns that the original traffic study and analyses were based on unknown assumptions. The new traffic study was completed after the I-10 widening was opened so that true numbers could be used for the analysis and observations could be made to see if the expected effects materialized as described in the report. The Baton Rouge General Ascension Neighborhood Hospital and the Bluff Primary School had also been opened, and new traffic counts gathered in November 2020 included the traffic generated by these developments.
Observations verified that the widening has increased the capacity and reduced congestion on the interstate, therefore allowing the ramp weave maneuver, which was previously causing spillback and blockage into the study area, to be corrected. Therefore, the assumptions made in the original traffic study were validated.
A review of the updated capacity analyses for the intersection of LA 73 and White Rd/Duplessis Rd indicates that the subdivision will add minimal delay to the intersection. The overall intersection is expected to continue to operate with acceptable levels of service for all scenarios. No geometric modifications are recommended. The eastbound and westbound approaches currently operate at LOS E and will continue to do so with the new development in place. In order to raise the LOS to D on these approaches, timing optimization of the traffic signal can be made. Whether or not signal optimization takes place, the overall LOS of the interstate is acceptable during both peak hours. Table 7 and Table 8 show the results with and without signal changes.
A review of the updated capacity analyses results for the intersection of LA 73 at Post Office Road/Alice Braud Lane indicates that the subdivision is not expected to cause significant impacts on the intersection during the morning and afternoon peak hours. The overall intersection is expected to continue to operate with acceptable levels of service for all scenarios. No geometric modifications are recommended. The westbound approach operates at LOS E and will continue to do so with the new development in place. In order to raise the LOS to D on this approach, traffic signal optimization can be performed. Whether or not signal optimization takes place, the overall LOS of the intersection is acceptable during both peak hours. Table 9 and Table 10 show the results with and without signal changes.
The traffic signal adjustments in the analyses for the analyses for Table 7 and Table 9 that would allow all approaches at these two intersections to operate at LOS D or better are optimizations that are typical signal adjustments that DOTD makes on a regular basis at signals on state-maintained highways, often in response to traffic studies performed for new developments. However, with the addition of the trips from the new subdivision, no significant increases in delay will occur even if no changes are made to these traffic signals by DOTD (Table 8 and Table 10). This is because the signals that exist are “actuated” signals. Actuated signals determine the timing and sequence of traffic movement based on what vehicles are detected at the intersection. When more vehicles are added to an approach of an intersection, the signal will give that approach additional green time, which is why some approaches see a slight decrease in delay once more vehicles are added to certain approaches. The overall LOS at both intersections will continue to be above LOS D for both peak hours, whether the timing changes are implemented or not. By making changes to allow the minor approaches to operate at LOS D, the overall LOS of the intersection is actually decreased because more vehicles are experiencing additional delay on the major approaches. This is the reason that Overall LOS is used by traffic engineers to evaluate effects of additional traffic at a signalized intersection, rather than the LOS of individual approaches.
A review of the updated capacity analysis results for the intersection of LA 73 and LA 621 indicates that now that the I-10 widening is complete and open, the overall intersection operates with acceptable levels of service for all scenarios. While the eastbound approach does currently operate at LOS E, the additional traffic will not increase the existing delay. While signal timing adjustments could be made to raise this approach to LOS D, this signal is coordinated with adjacent signals at I-10 ramps and C. Braud Road. Any signal timing adjustments would need to be coordinated with changes at the other signals as well. With the addition of the trips from the new subdivision, no significant increases in delay will occur if no changes are made to this actuated traffic signal by DOTD. Timing adjustments to raise the eastbound approach to LOS D are not recommended at this intersection. No geometric modification is recommended. Table 11 and Table 12 show the results with and without signal changes.
Table 11 and Table 12 both show the intersection of LA 73 and LA 621 will operate at an overall LOS C for both AM and PM peak hours with the proposed development in place. This is the same conclusion as the analyses results shown in Table 6, which were based on engineering assumptions made in the original study. Therefore, the engineering assumptions made in original traffic study and additional analyses requested by the ERA have been validated by the updated traffic study.
This document is submitted respectfully by:
Rebecca B. Lala P.E., PTOE
Quality Engineering & Surveying Traffic Engineer