ST9904-CS001107Grapevine SH 121 I SH 114 BIS Alternative Schematic Development
AGENDA
1. Introduction - Robert Jenkins
2. Traffic Volumes - Don Szczesny
3. Analyses - Matt Brown
A. Level-of-Service
B. Simulation
4. Discussion
PARSONS
TECHNICAL MEMORANDUM
To: Jerry Hodge
From: Robert W. Jenkins, P.E.
Parsons Transportation Group Inc.
Date: November 7, 2000
RE; SH 114/121 MIS Alternative Schematic Development
DRAFT
Introduction and Background
The Cities of Grapevine and Coppell expressed concerns about the access afforded by the
proposed SH 114/SH 121 MIS schematic that was presented by HDR this past May.
Parsons Transportation Group Inc. (PTG) was asked by the Cities to investigate
alternatives to provide better access in the corridor between Bus 114 (LP 382) and the
Grapevine Mills Parkway (FM 2499)/Grapevine Mills Blvd. North/Sandy Lake Road area.
It was agreed that TxDOT would furnish a revised assignment calibrated to reflect the
recent growth is this corridor; and that this assignment would be the basis for analyzing
alternative configurations. TxDOT staffing availability in Austin precluded upgrading the
assignment. As a result, PTG worked with NCTCOG to develop an assignment for the
alternative schematic. Peak hour assignments were developed and analyzed in
developing and refining alternative geometrics.
Although the assignment process appeared to amply address the important connections in
this segment of the corridor, some inaccuracies are noted as follows. The addition of an
arterial connection between SH 26 at Fairway and Royal Lane did not generate traffic
along the frontage road from Bass Pro; in reality we would some traffic to use this
connection. Likewise, very Iow peak volumes were assigned to the connection from the
Airport to LP 382. With the impending development like the Opryland Hotel, use of this
route seems reasonable.
This analysis effort supports the use of the alternative-interchanging concept to provide
better access in this corridor. We recommend that it be the basis for preliminary design in
lieu of the MIS concept for this part of the corridor.
Traffic Volumes
To determine the traffic volumes expected in Year 2020, PTG requested the assistance of
the North Central Texas Council of Governments (NCTCOG). NCTCOG provided PTG a
previously coded network of the study section for modification. PTG modified the network
to reflect the proposed changes in the freeway section. After the coded network was
revised, NCTCOG proceeded to run the model and produced Year 2020 AM and PM peak
hour model assignments. These traffic volumes were then checked for reasonableness
and adjusted, if necessary. These volumes became the basis for the level-of-service
analyses and the CORSIM simulation runs that were performed.
To further refine the LOS analyses and CORSIM simulation, PTG determined the weaving
movements in the major weaving areas. To determine the weaving movements, the
existing traffic counts (traffic counts previously collected by Grapevine and/or TxDOT)
were used and the percentages calculated were applied to the projected traffic volumes.
Traffic Analyses
Planning level traffic analysis was conducted of the proposed alternative using procedures
outlined in the 1997 Highway Capacity Manual. The purpose of the analysis was to
determine, on a preliminary basis, the number of traffic lanes along each ramp and
freeway section needed to accommodate year 2020 traffic volumes. Once the basic
number of lanes was determined for the alternative, the geometry was reviewed and
preliminary lane designations were made (through and auxiliary lanes). The lane
designations were selected to maximize freeway operations for the given cross-section.
The following Table summarizes the maximum allowable flow rate for basic freeway
segments for each combination of HCM level-of-service and number of lanes. This table
was used to determine the basic number of lanes to provide, at a minimum, a level-of-
service E during year 2020 peak traffic hours.
Table 1
Maximum Flow Rate by Number of Lanes and Level-of-Service Criteria
1997 Highw~ ! Capacity Manual
No of Lanes Maximum Flow Rate* (pcph) for stated LOS
LOS A LOS B LOS C LOS D LOS E
2 1,190 1,910 2,850 3,660 4,330
3 1,790 2,870 4,280 5,490 6,500
4 2,390 3,830 5,710 7,320 8,670
5 2,990 4,790 7,140 9,150 10,830
6 3,590 5,750 8,560 10,980 13,000
*Assumes level terrain, 6% trucks/buses, 65 mph free-flow speed, 0.95 PHF.
Detailed traffic analysis was conducted to evaluate the geometry and lane assignments
determined during the planning phase. Based upon the traffic forecasts, the critical time
period of analysis was determined to be the AM peak period in the southbound direction.
A CORSIM model was developed of the AM peak period that included southbound 121
from the Sandy Lake on-ramp to the mainline split between southbound 121 and Highway
114/DFW airport. Southbound FM 2499 was also included from the on-ramp at Stars and
Stripes to its junction with southbound 121.
CORSIM is a microscopic simulation tool developed for the Federal Highway
Administration that models individual vehicle and driver characteristics. Each vehicle in
the traffic stream is assigned a vehicle type having performance characteristics ranging
from a high-performance auto to a heavy commercial truck. Based upon the vehicle type,
allowable acceleration and deceleration ranges are assigned to the vehicle. Drivers are
assigned an aggressiveness rating ranging from passive to aggressive. A driver's
aggressiveness affects its tolerance to lane changing and car following under prevailing
traffic conditions. Geometry is explicitly modeled, including multiple destination lanes and
acceleration/deceleration lanes.
The advantage of a simulation model is that it permits a more thorough and therefore
accurate analysis of complex traffic operations such as those found within weaving and
merge/diverge areas. It also reports detailed measures-of-effectiveness (MOEs) such as
density and speed that assist in the evaluation of traffic operations.
A number of simulation runs were conducted of the alternative under year 2020 AM and
PM peak hour conditions. Where the simulation results indicated that operational
problems existed, improvements were made to the alternative's geometry to mitigate the
problem. Several iterations of geometry and simulation were needed until an acceptable
level-of-service was achieved for all segments. A summary of the AM and PM simulation
results is contained in Tables 2 and 3.