17 Case Study: Tennessee SR-109

17.1 Route Description

SR-109 is a rural highway approximately 30 miles east of Nashville, TN. It runs primarily north-south between I-65 (at the north, 36.63005995380142, -86.57224654711138) and I-840 (at the south, 36.13174756486397, -86.40076816231978). The route is shown in Figure 17.1, as well as the linked online map. It is approximately 40 miles in length. More background information about this route can be found in a report that documents an access management study for this corridor (Gresham Smith and Partners 2016).

Figure 17.1: Tennessee SR-109 Study Route Boundaries. Source: Map data from ©OpenStreetMap

17.2 Level of service

The level of service analysis focuses on the section of SR-109 between I-65 and the southern end of the city of Gallatin. This section is approximately 22 miles in length. The selected route is mostly contained within Sumner country. The northern terminus of the selected route, where it intersects with I-65, is just across the border of Robertson county. The southern portion of SR-109, not included in this analysis, is contained within Wilson county. The route is shown in Figure 17.2, as well as the linked online map.

Figure 17.2: Tennessee SR-109 LOS Study Route Boundaries. Source: Map data from ©OpenStreetMap

Segmentation

This route consists of sections of multilane highway, two-lane highway, with passing constrained and passing zone segments, several isolated signalized intersections, an arterial section through the small town of Portland, and several interchange ramp connections.

The general geometric configuration of the roadway through this section of SR-109 is as follows (from south to north):

• multilane highway in rural developed area (city of Gallatin), S Water Ave - Scotty Parker Rd/Old Hwy 109 N, approximately 5.93 mi
  
• multilane highway in rural undeveloped area, intersection with SR-52, approximately 11.38 mi
  
• arterial through city of Portland, SR-52 - Morningside Dr, approximately 1.02 mi
  
• two-lane highway in rural undeveloped area, Morningside Dr - Magnolia Springs Rd, approximately 2.84 mi
  
• multilane highway in rural undeveloped area, Magnolia Springs Rd - I-65 on-ramp, approximately 1.04 mi

The following photos⁵ illustrate the variety of geometric configurations found along this route. Note that these photos are from a vehicle traveling in the southbound direction of the route.

Figure 17.3: Tennessee SR-109, two-lane highway cross-section, north of Kirby Dr

Figure 17.4: Tennessee SR-109, transition from two-lane to multilane highway, at Morningside Dr

Figure 17.5: Tennessee SR-109, signalized intersection within city of Portland, at College St, no left-turn bays

Figure 17.6: Tennessee SR-109, signalized intersection within city of Portland, at Market St, no left-turn bays

Figure 17.7: Tennessee SR-109, multilane highway with TWLTL, curb and gutter, city of Portland, south of Village Dr

Figure 17.8: Tennessee SR-109, multilane highway with grass median, no curb and gutter, city of Portland, south of Academy Rd

Figure 17.9: Tennessee SR-109, multilane highway with ieft-turn and u-turn treatments, south of Scotty Parker Rd/Old Hwy 109 N

Figure 17.10: Tennessee SR-109, exit ramp at diamond interchange, W Albert Gallatin Ave

Figure 17.11: Tennessee SR-109, multilane highway with ieft-turn treatment, at Wedgewood Dr

Figure 17.12: Tennessee SR-109, multilane highway with TWLTL, no curb and gutter, north of Academy Rd

The final segmentation is shown in Table 17.1. The total length is 22.386 mi.

Table 17.1: Table 17.2: Final Segmentation for Tennessee SR-109 Study Route

Seg ID	From/To	Hwy Seg Type	Two-Lane Seg Type	# Dir Lanes	Length (ft)	Length (mi)	Terrain	Vertical Align Class	Posted Speed (mi/h)
1	Start Point - Clear Lake Meadows Dr.	MultilaneHwy	—	2	1853	0.3509	Level	NA	55
2	Clear Lake Meadows Dr. - Ryan Ave.	MultilaneHwy	—	2	4444	0.8417	Level	NA	55
3	Ryan Ave - Hancock St	MultilaneHwy	—	2	1517	0.2873	Rolling	NA	55
4	Hancock St.	SigIntersection	—	2	800	0.1515	NA	NA	55
5	Hancock St. - Nashville Turnpike Off Ramp	MultilaneHwy	—	2	2440	0.4621	Rolling	NA	55
6	Nashville Turnpike Off Ramp - Nashville Turnpike On Ramp	MultilaneHwy	—	2	897	0.1699	Rolling	NA	55
7	Nashville Turnpike On Ramp - Long Hollow Pike Off Ramp	MultilaneHwy	—	2	1430	0.2708	Rolling	NA	55
8	Hollow Point Pike Off Ramp - Hollow Point Pike On Ramp	MultilaneHwy	—	2	2196	0.4159	Rolling	NA	55
9	Hollow Point Pike On Ramp - Red River Road	MultilaneHwy	—	2	3303	0.6256	Level	NA	55
10	Red River Rd.	SigIntersection	—	2	300	0.0568	NA	NA	55
11	Red River Rd. - Albert Gallatin Ave Off Ramp	MultilaneHwy	—	2	3369	0.6381	Level	NA	55
12	Albert Gallatin Ave Off Ramp - Albert Gallatin Ave On Ramp	MultilaneHwy	—	2	1595	0.3021	Rolling	NA	55
13	Albert Gallatin Ave On Ramp - Old Douglas Rd.	MultilaneHwy	—	2	5377	1.0184	Rolling	NA	55
14	Old Douglas Rd - Old Hwy 109 N (Before)	MultilaneHwy	—	2	1340	0.2538	Rolling	NA	55
15	Old Hwy 109 N (End) - S Tunnel Rd	MultilaneHwy	—	2	16760	3.1742	Rolling	NA	55
16	S Tunnel Rd - Scotty Parker Rd	MultilaneHwy	—	2	8175	1.5483	Rolling	NA	55
17	Scotty Parker Rd - Old Tennessee 109	MultilaneHwy	—	2	847	0.1604	Rolling	NA	55
18	Old Tenessee Rd - Hollis Chapel Rd	MultilaneHwy	—	2	1182	0.2239	Rolling	NA	55
19	Hollis Chapel Rd - Roberts Rd	MultilaneHwy	—	2	2130	0.4034	Level	NA	55
20	Roberts Rd - Boiling Springs Rd	MultilaneHwy	—	2	8130	1.5398	Level	NA	55
21	Boiling Springs Rd - N Centerpoint Rd	MultilaneHwy	—	2	3505	0.6638	Rolling	NA	55
22	N Centerpoint Rd - PSL Decrease (55 to 45)	MultilaneHwy	—	2	5545	1.0502	Rolling	NA	55
23	PSL Drop (55 to 45) - SR-76	MultilaneHwy	—	2	2880	0.5455	Rolling	NA	45
24	SR-76/Fountainhead Rd	SigIntersection	—	2	300	0.0568	NA	NA	45
25	SR-76 - Longview Dr.	MultilaneHwy	—	2	4040	0.7652	Level	NA	45
26	Longview Dr.	SigIntersection	—	2	1400	0.2652	NA	NA	45
27	Longview Dr - SR-52	MultilaneHwy	—	2	5715	1.0824	Level	NA	45
28	SR-52	SigIntersection	—	2	225	0.0426	NA	NA	45
29	SR-52 - Morningside Dr	Arterial	—	2	5405	1.0237	NA	NA	30
30	Morningside Dr - Oakhill Dr	TwoLaneHwy	PassingConstrained	1	3633	0.6881	NA	1	45
31	Oakhill Dr - National Barn Company Driveway	TwoLaneHwy	PassingZone	1	2290	0.4337	NA	1	45
32	Kirby Dr	SigIntersection	—	1	1515	0.2869	NA	NA	45
33		TwoLaneHwy	PassingZone	1	1664	0.3152	NA	1	45
34		TwoLaneHwy	PassingConstrained	1	1170	0.2216	NA	1	45
35	Kenwood Dr	TwoLaneHwy	PassingZone	1	1264	0.2394	NA	1	45
36	Kenwood Dr - Woods Dr	TwoLaneHwy	PassingConstrained	1	970	0.1837	NA	1	45
37	Woods Dr - PSL Decrease (45 to 40)	TwoLaneHwy	PassingZone	1	1497	0.2835	NA	1	45
38	Magnolia Springs Rd	TwoLaneHwy	PassingConstrained	1	1590	0.3011	NA	1	40
39	Magnolia Springs Rd - Hwy 31 W	MultilaneHwy	—	2	3452	0.6538	Level	NA	40
40	Hwy 31 W	SigIntersection	—	2	425	0.0805	NA	NA	40
41	Hwy 31 W - I-65 (End Point)	MultilaneHwy	—	2	1628	0.3083	Level	NA	40

The vertical alignment generally ranges from level to rolling terrain. NA = Not Applicable. The changes in terrain along the route are shown in Table 17.3

Table 17.3: Terrain changes for Tennessee SR-109 Study Route

Segment Range	Terrain	Approximate Starting Location
1-2	Level	Start of route
3-8	Rolling	Ryan Rd
9-11	Level	Long Hollow Pike on-ramp
12-18	Rolling	Albert Gallatin Ave
19-20	Level	Holis Chapel Rd End
21-23	Rolling	Boiling Spring Rd
24-41	Level	SR-76 intersection

The screen captures of the graphical representation of the segmentation from Google Earth are not included here, as was done for the Oregon US-20 case study. Instead, the reader is referred to the KML file, at (https://github.com/NCHRP-08-135/Resources).

Discussion

Although there are not many changes to the number of lanes along this route, there are a relatively high number of segments because of changes in posted speed limit, terrain, signalized intersections, locations where significant changes in traffic volume occur (such as interchanges), and several changes in passing restrictions along the two-lane highway portion of the route.

Segments 17 and 18 are relatively short, 847 ft and 1182 ft, respectively. This is to accommodate changes in demand volumes. The intersections separating these segments provide separate left-turn bays.

The intersection at Kirby Dr (36°36’13.10”N, 86°32’13.06”W) is within a stretch of two-lane highway. Starting 645 ft upstream of the stop bar, the centerline striping changes from dashed yellow to solid yellow. Likewise, the solid yellow striping continues for 755 ft downstream of the intersection area before changing to dashed yellow. These three distances, 645 ft (upstream approach), 115 ft (intersection area), 755 ft (downstream approach) are combined together to represent the signalized intersection segment. If the directly adjacent upstream and downstream distances corresponding to the dashed yellow striping were coded as separate passing constrained segments, these segments would would be assigned a negative adjusted length because the calculated upstream and downstream influence area distances would exceed these passing constrained distances.

A posted speed limit (PSL) reduction (45 to 40 mi/h) occurs in Segment 38. Normally, a new segment would be started at the point of a PSL change; however, the passing zone from this point only continues for approximately 310 ft before changing to a passing constrained zone. Thus, this additional 310 ft of passing zone is included as part of Segment 38. From a practical perspective, drivers would treat this continuation of the passing zone beyond the PSL as part of the same stretch of passing zone upstream of the PSL change.

The arterial segment (29) ends at Morningside Dr, where two directional travel lanes transition to a single lane. At this point, a two-lane highway passing constrained segment begins. However, the arterial posted speed limit of 30 mi/h continues for approximately 590 ft after the end of the arterial segment before changing to 45 mi/h. Because this is a relatively short distance, this short stretch of passing constrained roadway at 30 mi/h is part of Segment 30, for which the remaining approximately 3040 ft of passing constrained roadway is signed for a speed limit of 45 mi/h.

Segmentation through town of Portland, TN

The arterial segment through the town of Portland consists of the following sub-segments:

#	Cross Street Boundaries	# Dir Lanes	Length (ft)	Cross Street Width	Posted Speed (mi/h)
1	SR-52 - Market St	2	1065	75	30
2	Market St - McGlothin St	2	600	105	30
3	McGlothin St - College St	2	315	130	30
4	College St - Morningside Dr	2	3055	60	30

Traffic Data

Traffic data were obtained from the Tennessee Department of Transportation (TDOT) Transportation Data Management System.

There are nine sensor locations along highway limits for the LOS analysis, as summarized in Table 17.4. Note that detectors are listed in order of northern most to southern most location along the route.

Table 17.4: SR-109 Sensor Locations

Location ID	Location Description	Approx. Location	Sensor Type
83000133	SR109 North Portland	36.60000902671167, -86.53358761270735	Axle/Tube
83000189	SR109 Portland	36.579966780040124, -86.51573967563627	Axle/Tube
83000018	SR109 Portland	36.564557580338835, -86.51077679030027	ATR Class
83000268	SR109 SR-109	36.52242136573732, -86.5098920651874	Axle/Tube
83000267	SR109 SR-109	36.47830136284571, -86.4773957598811	Axle/Tube
83000197	SR109 Gallatin	36.412250456982775, -86.47193224334912	Axle/Tube
83000196	SR109 Gallatin	36.393181597974774, -86.47892312861133	Axle/Tube
83000195	SR109 Gallatin	36.38150742324095, -86.47010146756689	Axle/Tube
83000194	SR109 Gallatin	36.36778239584337, -86.45912407493229	Axle/Tube

Table 17.5 summarizes the most recently available values for annual average daily traffic (AADT), percentage of traffic volume occurring in the peak hour of the day (K), and percentage of traffic volume traveling in the peak direction of the peak hour (D). The values shown in this table are for the most recent years for which actual field measurements were taken, not values estimated from growth projections.

Table 17.5: SR-109 Sensor Data – K, D, AADT

Location ID:	Year	K (%)	D (%)	AADT (veh/day)
83000133	2022	8	65	15,415
83000189	2022	8	54	19,423
83000018	2022	9	50	15,956
83000268	2020	10	57	10,990
83000267	2021	9	57	14,719
83000197	2022	9	58	19,048
83000196	2022	9	57	25,048
83000195	2022	9	58	34,979
83000194	2021	8	63	25,734

The PM peak hour volumes generally occurred between the hours of 4-6 PM, with the highest hour typically being 5-6 PM. Again, the values shown are generally based on the most recent field measurements. If multiple measurements were taken during a given year, common dates across detectors were selected if possible. For example, if one sensor had counts taken during May and September of a given year and another sensor had counts taken during just May of the same year, the counts for May would be selected for both sensors. Furthermore, if multiple days of data were available in the same month of the same year, the same day of the week would be chosen across the sensors if possible.

The PHF values range from approximately 0.87-0.96. Because specific traffic peaking times will likely vary over a route of this length, as well as to not over complicate the process of ‘conserving’ vehicles throughout the full length of the route when setting traffic demand values, a single PHF value is used. For this analysis, an approximate mid-range value of 0.91 is used. Consequently, this value effectively increases the demand flow rate for analysis purposes by 10%. The original PM peak hour volumes and corresponding values as adjusted by the PHF (rounded to the nearest 50 veh) are shown in Table 17.6. The PHF values are then set to a value of 1.0 in the input data settings.

Table 17.6: SR-109 Sensor Data – Northbound PM Peak Hourly Volumes

Location ID:	Count Date	Hourly Volume (veh/h)	Adjusted Hourly Volume (veh/h)
83000133	Mon 5/23/2022	652*	700
83000189	Wed 6/8/2022	826	950
83000018	Thu 6/9/2022	779	850
83000268	Thu 5/14/2020	677	750
83000267	Thu 5/6/2021	906	1,000
83000197	Wed 5/4/2022	1,142	1,250
83000196	Wed 5/4/2022	1,429	1,550
83000195	Wed 5/4/2022	1,887	2,050
83000194	Wed 5/19/2021	909	1,000

• Data not available, so value imputed based on AADT relationship to adjacent detector

With a total of nine sensor locations spatially distributed across 22 miles of highway, determining locations to affect volume changes along the route is a very approximate process. To inform this process, satellite photography of the route and its surrounding area was reviewed. More major intersecting roadways–indicated by number of lanes, turning movement accommodation from the major roadway, and/or density of land use accessed by intersecting road within immediate area–were typically chosen as the locations to implement the volume changes.

For two-lane highway segments, the opposing direction volume is needed. Based on Detector 83000133 (in Segment 34), the one sensor within the two-lane highway portion of the route, the D factor value of 65% was used to set the opposing direction volume for the PM peak hour; that is, opposing direction volume = peak direction volume / 0.65 \(-\) peak direction volume (rounded to the nearest 50 veh).

The assignment of volumes (rounded to nearest 50 vehicles) to segments, and locations where volume changes are implemented are as follows:

• Sensor 83000194 is located within Segment 2. Its volume of 1000 veh/h is applied to Segments 1-5.
• Sensor 83000195 is located within Segment 7. There is a net volume increase of 1050 veh/h at this sensor. This net volume change of +1000 is implemented by assigning an exit volume of -250 at the Nashville Pike interchange (start of Segment 6) and an entrance volume of +1300 at the Nashville Pike interchange (start of Segment 7).
  • Volume in Segment 6 is 750 veh/h.
  • Volume in Segment 7 is 2050 veh/h.
• Sensor 83000196 is located within Segment 9. There is a net volume decrease of 500 veh/h at this sensor. This net volume change of -500 is implemented by assigning an exit volume of -900 at the Long Hollow Pike interchange (start of Segment 8) and an entrance volume of +400 at the Long Hollow Pike interchange (start of Segment 9).
  • Volume in Segment 8 is 1150 veh/h.
  • Volume in Segment 9 is 1550 veh/h.
• Sensor 83000197 is located within Segment 13. There is a net volume decrease of 300 veh/h at this sensor. This net volume change of -300 is implemented by assuming a net volume change of -100 veh/h at the intersection with Red River Rd (Segment 10), assigning an exit volume of -300 at the Albert Gallatin Ave interchange (start of Segment 12) and an entrance volume of +100 at the Albert Gallatin Ave interchange (start of Segment 13).
  • Volume in Segment 10 is 1450 veh/h.
  • Volume in Segment 11 is 1450 veh/h.
  • Volume in Segment 12 is 1150 veh/h.
  • Volume in Segment 13 is 1250 veh/h.
• Sensor 83000267 is located within Segment 16. There is a net volume decrease of 250 veh/h at this sensor. This net volume change of -250 is implemented by assuming a net volume change of -150 veh/h at the intersection with Old Hwy 109 N/Scotty Parker Rd (start of Segment 15) and a net volume change of -100 veh/h at the intersection with S Tunnel Rd (start of Segment 16).
  • Volume in Segment 14 is 1250 veh/h.
  • Volume in Segment 15 is 1100 veh/h.
  • Volume in Segment 16 is 1000 veh/h.
• Sensor 83000268 is located within Segment 21. There is a net volume decrease of 250 veh/h at this sensor. This net volume change of -250 is implemented by assuming a net volume change of -150 veh/h at the intersection with Roberts Rd (start of Segment 20) and a net volume change of -100 veh/h at the intersection with Boiling Springs Rd (start of Segment 21).
  • Volume in Segment 17 is 1000 veh/h.
  • Volume in Segment 18 is 1000 veh/h.
  • Volume in Segment 19 is 850 veh/h.
  • Volume in Segment 20 is 850 veh/h.
  • Volume in Segment 21 is 750 veh/h.
• Sensor 83000018 is located within Segment 27. There is a net volume increase of 100 veh/h at this sensor. This net volume change of +100 is implemented by assuming a net volume change of +100 veh/h at the intersection with SR-76 intersection (Segment 24).
  • Volume in Segment 22 is 750 veh/h.
  • Volume in Segment 23 is 750 veh/h.
  • Volume in Segment 24 is 750 veh/h.
  • Volume in Segment 25 is 850 veh/h.
  • Volume in Segment 26 is 850 veh/h.
  • Volume in Segment 27 is 850 veh/h.
• Sensor 83000189 is located within Segment 29. There is a net volume increase of 100 veh/h at this sensor. This net volume change of +100 is implemented by assuming a net volume change of +100 veh/h at the intersection with SR-52 intersection (Segment 28).
  • Volume in Segment 28 is 850 veh/h.
  • Volume in Segment 29 is 950 veh/h.
• Sensor 83000133 is located within Segment 31. There is a net volume decrease of 250 veh/h at this sensor. This net volume change of -250 is implemented by assuming a net volume change of -250 veh/h at the intersection with Freedom Dr (within arterial segment, Segment 29)
  • Volume in Segment 30 is 700 veh/h. Opposing direction volume is 400 veh/h.
  • Volume in Segment 31 is 700 veh/h. Opposing direction volume is 400 veh/h.
• Volume in Segments 32-41 is 700 veh/h. For segments 33-38, opposing direction volume is 400 veh/h.

Vehicle classification data are available from five sensor locations (83000133, 83000018, 83000267, 83000196, 83000195). The truck percentages for the hour of 5:00-6:00 PM for the northbound direction, rounded to the nearest integer value, and the segments to which they are applied, are given in Table 17.7.

Table 17.7: SR-109 Sensor Data – PM Peak Hour Truck Percentages

Location ID:	Date	Small Truck %	Large Truck %	Total Truck %	Applicable Segment(s)
83000195	4/22/2020	2	6	8	1-8
83000196	4/16/2020	1	3	4	9-14
83000267	6/9/2022	1	5	6	15-23
83000018	6/9/2022	1	13	14	24-28
83000133	5/20/2019	2	7	9	29-41

Note that for all but the multilane highway segment type, only the total truck percentage is used in the corresponding segment analysis methodology calculations. For multilane highways, the separate percentages for small and large trucks are used when performing an analysis for a segment with a specific grade (i.e., explicit consideration of grade percentage and length of grade). For a segment that using the ‘general terrain’ classification (i.e., level, rolling), just the total truck percentage is utilized.

The Albert Gallatin Ave interchange may not have been open at the time of these volume counts. Satellite photo shows construction vehicles on connecting road. This interchange probably changed turning volumes at Red River Rd (to the south) and Old Hwy 109 N/Scotty Parker Rd (to the north)

Additional Data Inputs for Intersections

Signalized Intersections

The following is assumed about turning movement volumes from SR-109 onto cross streets:

• 10% total (5% left, 5% right) at intersections with minor roads
• 20% total (10% left, 10% right) at intersections with major roads
• If the intersection is not at a location where a net volume difference is implemented (according to information above), then it is assumed that a volume equal to the volume turning from the major roadway enters the major roadway from the cross street.

The route includes isolated signalized intersections at the following cross streets:

• Hancock St
• Red River Rd
• SR-76/Fountainhead Rd
• Longview Rd
• SR-52
• Kirby Dr (essentially 3-leg)
• Highway 31W

All of these intersections include two through lanes and a left turn bay for the major street approaches. The major street approaches at the intersections with Hancock St, SR-52, and Highway 31W also include a right turn bay.

The intersection with Kirby Dr. is, for practical purposes, a 3-leg intersection. The eastbound leg just serves as an access to a very small business; thus, the traffic volume into and out of this leg of the intersection is negligible for analysis purposes.

The route includes several intersections that are included within the arterial segment (29), which runs through the town of Portland. These intersections are located at the following cross streets:

• Market St
• McGlothin St
• College St (3-leg)

All of these intersections include two through lanes for the major street approaches. However, none of them include left or right turn bays.

Specific signal timing data were not available, so the following assumptions were made.

Isolated Intersections:

• Cycle length: 90 seconds
• Effective green/cycle length ratio: 0.44
• Arrival type: 3 (random arrivals)

Arterial intersections (no turn bays)

• Cycle length: 60 seconds
• Effective green/cycle length ratio: 0.48 (higher than for isolated intersections because of lack of exclusive left turn phasing)
• Arrival type: 4 (favorable progression)

Two-Way Stop Controlled (TWSC) Intersections

These locations are typically used as segment break points to allow for a change in traffic volume. However, because it assumed that the major through movement does not incur delay due to any turning vehicles, no specific intersection analysis is required at these locations. For locations where intersection geometry upgrades are necessary because of delay due to turning movements, the reader is referred to some recommended studies listed in Appendix A.

All-Way Stop Controlled (AWSC) Intersections

No AWSC intersections are present along this route.

Roundabout Intersections

No roundabout intersections are present along this route.

Results

For multilane highway segments, the base free-flow speed (BFFS) was set per the HCM guidance; that is BFFS = Posted Speed Limit (PSL) + 5, for posted speeds \(\leq\) 45 mi/h and BFFS = PSL + 7 for posted speeds > 45 mi/h.

The segment LOS results are shown in Tables 17.8 and 17.10.

Table 17.8: Table 17.9: SR-109 Segment LOS Analysis Results, Part 1

Seg ID	Hwy Seg Type	Two-Lane Seg Type	# Dir Lanes	Length (mi)	Eff. Len. Upstream (mi)	Eff. Len. Downstream (mi)	Adj. Length (mi)	Posted Speed (mi/h)	Dir Volume (veh/h)	Opp Volume (veh/h)	PHF	Dir Truck%	Analysis Flow Rate (veh/h/ln)	Analysis Flow Rate (pc/h/ln)	Vertical Align Class	FFS (mi/h)	Free-Flow Trav Time (s)	Avg Speed (mi/h)	Avg Travel Time (s)
1	MultilaneHwy	—	2	0.3509000	—	—	0.3509	55	1000	—	1	8	1000	540	—	58.2000	21.71	58.20	21.71
2	MultilaneHwy	—	2	0.8417000	—	—	0.8417	55	1000	—	1	8	1000	540	—	57.5000	52.70	57.50	52.70
3	MultilaneHwy	—	2	0.2873000	—	—	0.2155	55	1000	—	1	8	1000	580	—	57.5000	13.49	57.50	13.49
4	SigIntersection	—	2	0.1515152	0.1475	0.2724	0.4200	55	1000	—	1	8	900	—	—	60.5000	24.99	32.84	46.04
5	MultilaneHwy	—	2	0.4621000	—	—	0.2654	55	1000	—	1	8	1000	580	—	57.5000	16.62	57.50	16.62
6	MultilaneHwy	—	2	0.1699000	—	—	0.1699	55	750	—	1	8	750	435	—	57.5000	10.64	57.50	10.64
7	MultilaneHwy	—	2	0.2708000	—	—	0.2708	55	2050	—	1	8	2050	1189	—	57.5000	16.95	57.50	16.95
8	MultilaneHwy	—	2	0.4159000	—	—	0.4159	55	1150	—	1	8	1150	667	—	57.5000	26.04	57.50	26.04
9	MultilaneHwy	—	2	0.6256000	—	—	0.5065	55	1550	—	1	4	1550	806	—	57.5000	31.71	57.50	31.71
10	SigIntersection	—	2	0.0568182	0.1475	0.267	0.4145	55	1450	—	1	4	1378	—	—	60.5000	24.66	27.31	54.64
11	MultilaneHwy	—	2	0.6381000	—	—	0.3996	55	1450	—	1	4	1450	754	—	57.5000	25.02	57.50	25.02
12	MultilaneHwy	—	2	0.3021000	—	—	0.3021	55	1150	—	1	4	1150	621	—	57.5000	18.91	57.50	18.91
13	MultilaneHwy	—	2	1.0184000	—	—	1.0184	55	1250	—	1	4	1250	675	—	57.5000	63.76	57.50	63.76
14	MultilaneHwy	—	2	0.2538000	—	—	0.2538	55	1250	—	1	4	1250	675	—	57.5000	15.89	57.50	15.89
15	MultilaneHwy	—	2	3.1742000	—	—	3.1742	55	1100	—	1	6	1100	616	—	57.5000	198.73	57.50	198.73
16	MultilaneHwy	—	2	1.5483000	—	—	1.5483	55	1000	—	1	6	1000	560	—	57.5000	96.94	57.50	96.94
17	MultilaneHwy	—	2	0.1604000	—	—	0.1604	55	1000	—	1	6	1000	560	—	57.5000	10.04	57.50	10.04
18	MultilaneHwy	—	2	0.2239000	—	—	0.2239	55	1000	—	1	6	1000	560	—	57.5000	14.02	57.50	14.02
19	MultilaneHwy	—	2	0.4034000	—	—	0.4034	55	850	—	1	6	850	450.5	—	57.5000	25.26	57.50	25.26
20	MultilaneHwy	—	2	1.5398000	—	—	1.5398	55	850	—	1	6	850	450.5	—	57.5000	96.40	57.50	96.40
21	MultilaneHwy	—	2	0.6638000	—	—	0.6638	55	750	—	1	6	750	420	—	57.5000	41.56	57.50	41.56
22	MultilaneHwy	—	2	1.0502000	—	—	1.0502	55	750	—	1	6	750	420	—	57.5000	65.75	57.50	65.75
23	MultilaneHwy	—	2	0.5455000	—	—	0.4663	45	750	—	1	6	750	420	—	49.5000	33.92	49.50	33.92
24	SigIntersection	—	2	0.0568182	0.1076	0.1894	0.2969	45	750	—	1	14	712	—	—	49.5000	21.60	26.02	41.07
25	MultilaneHwy	—	2	0.7652000	—	—	0.5251	45	850	—	1	14	850	484.5	—	49.5000	38.19	49.50	38.19
26	SigIntersection	—	2	0.2651515	0.1076	0.1894	0.2969	45	850	—	1	14	807	—	—	49.5000	21.60	25.33	42.21
27	MultilaneHwy	—	2	1.0824000	—	—	1.0782	45	850	—	1	14	850	484.5	—	49.5000	78.42	49.50	78.42
28	SigIntersection	—	2	0.0426136	0.0942	0.0489	0.1431	45	850	—	1	14	807	—	—	49.5000	10.40	16.60	31.01
29	Arterial	—	2	1.0236742	0	0	0.9748	30	800	—	1	9	950	—	—	37.3300	98.72	25.21	146.16
30	TwoLaneHwy	PassingConstrained	1	0.6881000	—	—	0.6881	45	700	400	1	9	700	—	1	51.0003	48.57	48.33	51.26
31	TwoLaneHwy	PassingZone	1	0.4337000	—	—	0.3980	45	700	400	1	9	700	—	1	51.0003	28.10	48.60	29.48
32	SigIntersection	—	1	0.2869318	0.1578	0.2016	0.3594	45	700	—	1	9	700	—	—	49.5000	26.14	17.66	73.26
33	TwoLaneHwy	PassingZone	1	0.3152000	—	—	0.2784	45	700	400	1	9	700	—	1	51.0003	19.65	48.61	20.62
34	TwoLaneHwy	PassingConstrained	1	0.2216000	—	—	0.2216	45	700	400	1	9	700	—	1	51.0003	15.64	48.36	16.50
35	TwoLaneHwy	PassingZone	1	0.2394000	—	—	0.2394	45	700	400	1	9	700	—	1	51.0003	16.90	48.61	17.73
36	TwoLaneHwy	PassingConstrained	1	0.1837000	—	—	0.1837	45	700	400	1	9	700	—	1	51.0003	12.97	48.36	13.68
37	TwoLaneHwy	PassingZone	1	0.2835000	—	—	0.2835	45	700	400	1	9	700	—	1	51.0003	20.01	48.61	21.00
38	TwoLaneHwy	PassingConstrained	1	0.3011000	—	—	0.3011	40	700	400	1	9	700	—	1	45.3003	23.93	42.90	25.27
39	MultilaneHwy	—	2	0.6538000	—	—	0.6059	40	700	—	1	9	700	381.5	—	44.5000	49.02	44.50	49.02
40	SigIntersection	—	2	0.0804924	0.0858	0.1255	0.2113	40	700	—	1	9	665	—	—	44.0000	17.28	21.22	35.84
41	MultilaneHwy	—	2	0.3083000	—	—	0.2255	40	700	—	1	9	700	381.5	—	44.5000	18.24	44.50	18.24

Table 17.10: Table 17.11: SR-109 Segment LOS Analysis Results, Part 2

Seg ID	% Followers	Density (pc/mi/ln)	Density (veh/mi/ln)	Follower Density (veh/mi/ln)	Adj. Follower Density (veh/mi/ln)	Avg. Thresh. Delay (s/veh)	Avg. Thresh. Delay (%)	Avg. FFS Delay (s/veh)	Avg. FFS Delay (%)	LOS	LOS Value	Demand/Capacity	Available Capacity (veh/h)	Momentum (vol x speed)	VMT	VHT	VHD
1	—	9.3	—	—	—	0.0	0.0	0.0	0.0	A	0.84	0.250	3007	58200.0	350.9	6.03	0.000
2	—	9.4	—	—	—	0.0	0.0	0.0	0.0	A	0.85	0.251	2981	57500.0	841.7	14.64	0.000
3	—	10.1	—	—	—	0.0	0.0	0.0	0.0	A	0.92	0.270	2707	57500.0	215.5	3.75	0.000
4	—	—	—	—	—	18.6	67.5	21.1	84.2	C	2.07	0.600	589	29554.1	378.0	11.51	5.263
5	—	10.1	—	—	—	0.0	0.0	0.0	0.0	A	0.92	0.270	2707	57500.0	265.4	4.62	0.000
6	—	7.6	—	—	—	0.0	0.0	0.0	0.0	A	0.69	0.202	2957	43125.0	127.4	2.22	0.000
7	—	20.7	—	—	—	0.0	0.0	0.0	0.0	C	2.33	0.553	1657	117875.0	555.1	9.65	0.000
8	—	11.6	—	—	—	0.0	0.0	0.0	0.0	B	1.09	0.310	2557	66125.0	478.3	8.32	0.000
9	—	14	—	—	—	0.0	0.0	0.0	0.0	B	1.43	0.375	2585	89125.0	785.0	13.65	0.000
10	—	—	—	—	—	27.5	101.4	30.0	121.5	C	2.66	0.870	199	37635.2	571.2	20.91	11.472
11	—	13.1	—	—	—	0.0	0.0	0.0	0.0	B	1.30	0.351	2685	83375.0	579.4	10.08	0.000
12	—	10.8	—	—	—	0.0	0.0	0.0	0.0	A	0.98	0.289	2831	66125.0	347.4	6.04	0.000
13	—	11.7	—	—	—	0.0	0.0	0.0	0.0	B	1.11	0.314	2731	71875.0	1273.0	22.14	0.000
14	—	11.7	—	—	—	0.0	0.0	0.0	0.0	B	1.11	0.314	2731	71875.0	317.3	5.52	0.000
15	—	10.7	—	—	—	0.0	0.0	0.0	0.0	A	0.97	0.287	2739	63250.0	3491.6	60.72	0.000
16	—	9.7	—	—	—	0.0	0.0	0.0	0.0	A	0.89	0.260	2839	57500.0	1548.3	26.93	0.000
17	—	9.7	—	—	—	0.0	0.0	0.0	0.0	A	0.89	0.260	2839	57500.0	160.4	2.79	0.000
18	—	9.7	—	—	—	0.0	0.0	0.0	0.0	A	0.89	0.260	2839	57500.0	223.9	3.89	0.000
19	—	7.8	—	—	—	0.0	0.0	0.0	0.0	A	0.71	0.210	3207	48875.0	342.9	5.96	0.000
20	—	7.8	—	—	—	0.0	0.0	0.0	0.0	A	0.71	0.210	3207	48875.0	1308.8	22.76	0.000
21	—	7.3	—	—	—	0.0	0.0	0.0	0.0	A	0.66	0.195	3089	43125.0	497.9	8.66	0.000
22	—	7.3	—	—	—	0.0	0.0	0.0	0.0	A	0.66	0.195	3089	43125.0	787.7	13.70	0.000
23	—	8.5	—	—	—	0.0	0.0	0.0	0.0	A	0.77	0.211	2804	37125.0	349.8	7.07	0.000
24	—	—	—	—	—	17.3	72.9	19.5	90.2	B	1.95	0.510	692	18529.5	211.4	8.12	3.853
25	—	9.8	—	—	—	0.0	0.0	0.0	0.0	A	0.89	0.243	2641	42075.0	446.3	9.02	0.000
26	—	—	—	—	—	18.5	77.7	20.6	95.4	C	2.04	0.570	597	20439.0	239.6	9.46	4.620
27	—	9.8	—	—	—	0.0	0.0	0.0	0.0	A	0.89	0.243	2641	42075.0	916.5	18.51	0.000
28	—	—	—	—	—	19.6	171.0	20.6	198.1	C	2.04	0.570	597	13399.9	115.4	6.95	4.620
29	—	—	—	—	—	23.3	19.0	47.4	48.1		1.46	0.000	0	23953.4	972.5	38.57	12.518
30	65.5	—	14.5	9.5	9.5	0.0	0.0	2.7	5.5	C	2.90	0.412	1000	33829.4	481.7	9.97	0.522
31	64.1	—	14.4	9.2	9.2	0.0	0.0	1.4	4.9	C	2.85	0.412	1000	34020.9	278.6	5.73	0.270
32	—	—	—	—	—	44.5	154.8	47.1	180.3	D	3.61	0.950	37	12362.7	251.6	14.24	9.162
33	65	—	14.4	9.4	9.4	0.0	0.0	1.0	4.9	C	2.87	0.412	1000	34026.7	194.9	4.01	0.188
34	67.9	—	14.5	9.8	9.8	0.0	0.0	0.9	5.5	C	2.97	0.412	1000	33848.6	155.1	3.21	0.166
35	65.3	—	14.4	9.4	9.4	0.0	0.0	0.8	4.9	C	2.88	0.412	1000	34028.2	167.6	3.45	0.161
36	67.9	—	14.5	9.8	9.8	0.0	0.0	0.7	5.5	C	2.97	0.412	1000	33848.6	128.6	2.66	0.138
37	64.9	—	14.4	9.4	9.4	0.0	0.0	1.0	4.9	C	2.87	0.412	1000	34026.4	198.5	4.08	0.191
38	68.3	—	16.3	11.2	11.2	0.0	0.0	1.3	5.6	D	3.23	0.412	1000	30030.5	210.8	4.91	0.260
39	—	8.6	—	—	—	0.0	0.0	0.0	0.0	A	0.78	0.202	2768	31150.0	424.1	9.53	0.000
40	—	—	—	—	—	16.8	88.5	18.6	107.3	B	1.86	0.450	820	14111.7	140.5	6.62	3.427
41	—	8.6	—	—	—	0.0	0.0	0.0	0.0	A	0.78	0.202	2768	31150.0	157.8	3.55	0.000

Table 17.12 provides the results for the individual subsegments within the arterial segment, again specific to the major street through movement.

Table 17.12: SR-109 Arterial Segment (29) LOS Analysis Results

#	Cross Street Boundaries	Analysis Flow Rate (veh/h)	Adj. Sat. Flow Rate (veh/h)	v/c	Control Delay (s/veh)
1	SR-52 - Market St	950	2433	0.814	11.9
2	Market St - McGlothin St	2433	0.814	11.9	16.5
3	McGlothin St - College St	2433	0.814	11.9	11.8
4	College St - Morningside Dr	NA	0.814	–	38.4

The facility LOS results are shown in Table 17.13

Table 17.13: SR-109 Facility Analysis Results

Performance Measure	Value
Free Flow Travel Time (s)	1511.1
Avg. Travel Time (s)	1745.66
Avg. Speed (mi/h)	46.17
VMT (veh-mi)	21488.24
VHT (veh-h)	454.152
Delay (veh-h)	56.833
Avg. FFS Delay (s/veh)	234.59
Avg. FFS Delay (%)	15.5
Avg. Threshold Delay (s/veh)	186.05
Avg. Threshold Delay (%)	12.3
LOS Score (Weighted-Travel Time)	1.478
LOS Constancy	0.601
LOS Adjustment Factor	1.08
LOS Score (Weighted-Travel Time and Adjusted)	1.596
Maximum d/c	0.95

Discussion

The overall facility LOS, based on a score of 1.596, is B. FFS and threshold delays, 15.5% and 12.3%, are relatively low. The several large signalized intersections and slow speed through the city of Portland are the primary contributors to an average speed of just 46 mi/h along this route.

Density values for the multilane highway segments generally range approximately from 7-12 veh/mi/ln, with LOS values of A and B. Volume to capacity ratios range approximately from 0.2-0.5. Density and follower density values for the two-lane highway segments are approximately 14 veh/mi/ln and 10 followers/mi/ln, with LOS values of C. Volume to capacity ratios are approximately 0.4.

The highest v/c ratios along the route generally occur at signalized intersection locations. The isolated signalized intersection results are extracted from the previous tables (17.8 and 17.10) and summarized as follows.

Seg ID	Control Delay (s/veh)	LOS	v/c
Segment 4 (Hancock St)	21.1	C	0.600
Segment 10 (Red River Rd.)	30.0	C	0.870
Segment 24 (SR-76)	19.5	B	0.510
Segment 26 (Longview Dr)	22.3	C	0.550
Segment 28 (SR-52)	20.6	C	0.570
Segment 32 (Kirby Dr)	29.3	C	0.850
Segment 40 (Hwy 31 W)	18.6	B	0.450

Additionally, the signalized intersections that are part of the arterial segment that runs through the town of Portland have delays of approximately 12 s/veh and \(v/c\) ratios of approximately 0.8 (also shown in Table 17.12). The \(v/c\) ratios are relatively high for these intersections because of the lack of exclusive left-turn bays and phasing. However, the control delay values are still relatively low due to the low traffic volumes. It should also be noted that the average speed for McGlothin St-College St is very low, despite relatively low control delay, due to the short link length (315 ft). The average speed for the entire arterial of 25.2 mi/h (LOS B), however, is still very reasonable for a section of roadway with a posted speed limit of 30 mi/h.

It also needs to be noted that this route has some characteristics that currently are not explicitly considered in the HCM urban streets analysis methodology. For example, much of this section is without curb and gutter and has essentially continuous frontage of business access. While the through traffic volumes are currently relatively low and the local businesses do not appear to be high traffic generators, should one or both of the traffic demand characteristics increase significantly, the lack of access management along this section of roadway could lead to some operational complications.

Furthermore, if there is any considerable growth in traffic demand in this area, the signalized intersections in the city of Portland will likely need to be upgraded to include left turn bays.

17.3 Reliability

The reliability analysis examines a 4-mile stretch of SR-109 in the rural town of Gallatin, Tennessee (see Figure 17.13). For the analysis, the facility was divided into 9 segments with each segment measuring approximately 0.3-0.6 miles in length.

https://www.google.com/maps/dir/36.3636023,-86.4408041/36.4036951,-86.4771867/@36.3885173,-86.4682693,2516m/data=!3m1!1e3!4m2!4m1!3e0

Figure 17.13: Reliability Analysis Section, Through Gallatin, TN, along SR-109. Source: Map data ©2022 Google

The facility was evaluated using five data analysis and visualization techniques that convert the raw speed and travel time data into charts and graphics for analysis and interpretation. These methods are described in Section 3.6).

Speed Heatmaps

Speed heatmaps are shown in Figure 17.14.

Figure 17.14: Speed Heatmaps

Speed Difference Heatmaps

Speed difference heatmaps shown in Figure 17.15.

Figure 17.15: Speed Difference Heatmaps

Box-and-Whisker Speed Plots

In the box-and-whisker weekly speed plots shown in Figure 17.16, speed (mi/h) is shown on the x-axis and time (in weeks) is shown on y-axis. The chart uses box-and-whisker plots to provide a summary of the weekly speeds for the analysis period.

Figure 17.16: Box-and-Whisker Weekly Speed Averages Plot

Speed Confidence Band

The speed confidence plots are shown in Figure 17.17.

Figure 17.17: Speed Confidence Bands

Travel Time Index

The TTI plots are shown in Figure 17.18.

Figure 17.18: Travel Time Index (TTI) Plots

Summary

This case study presented a data-driven approach to explore travel time reliability on a rural highway in Tennessee (i.e., SR-109) as it traverses the small town of Gallatin, three grade-separated interchanges, and rural areas. Five different visualization techniques were used to derive insights from higher-resolution vehicular probe data from INRIX XD. Segments outside Gallatin generally maintain free-flow speeds in the range of 50-60 mi/h. The interchanges are associated with some of the sharpest slowdowns, particularly in the PM peak period.

Over the course of the analysis year (October 2021-March 2022), this corridor did not experience a major disruption such as a natural disaster or a spike in demand. Seasonal slowdowns were noted over the winter months, from December 2021 through March 2022. These contributed to higher variability in travel times, as evidenced by the higher travel time indices.

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5. Photos courtesy of Scott Washburn.↩︎