1 intersection decision support: a cooperative approach summary of california path / uc berkeley ids...
TRANSCRIPT
![Page 1: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/1.jpg)
1
Intersection Decision Support: A Cooperative Approach
Summary of California PATH / UC Berkeley IDS Approach
![Page 2: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/2.jpg)
2
Outline
• Background• California Approach
– Work Plan– IDS Architecture
• Underpins California Concept of Operations
– Integrating the Driver• Aimed at IDS Alert System Design
– Alert System Design– Approach to Cooperative Systems
![Page 3: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/3.jpg)
3
Background
![Page 4: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/4.jpg)
4
Driver Decision Support Concept
• Emphasis on crossing-path collisions (both signalized and unsignalized)– 80% of intersection crashes (1998 GES)
• Concept and architecture accommodate other intersection crash types
• Find “best” combinations of vehicle and infrastructure responsibilities for diverse intersections
![Page 5: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/5.jpg)
5
California Areas of Interest
• LTAP/OD conflicts at signals (permissive, not protected)
• LTAP/LD conflicts at urban stop signs• Development of broad system architecture
applicable to all intersection crash types, using information from infrastructure and/or vehicles
• Development of wireless communications to connect infrastructure and vehicles
• Pedestrian and bicyclist conflicts
![Page 6: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/6.jpg)
6
California’s Primary Focus Area: LTAP/OD
• Features of California LTAP/OD Approach– Problem is gap decision in presence of oncoming
vehicles– For all traffic control devices: signal, stop sign, no
controls– High potential for initial infrastructure-based warning
application (“Driver Infrastructure Interface”, DII)• Takes advantage of left turn dynamics
– If designed correctly, DII for left turn driver is unambiguous• DII located at far side corner, where driver is likely to scan
– Cooperative (wireless) systems should enhance system
![Page 7: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/7.jpg)
7
Definition: Left Turn Across Path / Opposite Direction (LTAP/OD)
![Page 8: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/8.jpg)
8
Principal Expected Products
• Quantitative characterizations of intersection safety problems and turning behavior at intersections
• Recommended designs for infrastructure elements and systems (sensors and their locations, algorithms to use sensor outputs to alert drivers, and displays)
• Estimates of effectiveness of IDS in reducing intersection crashes
• Plans for public operational test(s) at selected intersections
![Page 9: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/9.jpg)
9
California Approach
![Page 10: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/10.jpg)
10
Schematic of California Approach
Design IDS Alert System
Characterize Intersection Driving• Roadside Observations• Driver Observations
Evaluate COTS Technologies
Develop Simulation Tool
Develop Wireless CommunicationSystem
Develop DII
Driving ExperimentsAt RFS Intersection
Implement RFSTest Intersection
![Page 11: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/11.jpg)
11
Task List and Status (1 of 2)
• Completed Tasks:– A. Problem Assessment
• Deliverables: Problem Assessment Reports– M. Mid-term demonstration
• Deliverable: Year 1 Report (in process)• Current Tasks:
– B. Characterize Intersection Driving• Deliverables: Pilot Feasibility Study, Extended Observation
Study, Field Collection, Lag Acceptance Reports (early 05)– C. Wireless Communication Development
• Deliverables: Protocol Design Report (July 04), Communication Prototype and Performance Analysis Report (early 05)
– D. Driver-Infrastructure-Interface Usability• Deliverables: DII Usability Report (early 05), DII
Alternatives Report (early 05)– E. Develop Warning System
• Deliverable: Warning System (early 05)
![Page 12: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/12.jpg)
12
Task List and Status (2 of 2)
• Current Tasks (Cont’d)– S1. Develop Simulation Tool
• Deliverables: Tool (Nov 04)
– S2. Evaluate COTS Technologies• Deliverables: COTS System Downselect (Sep 04),
Test Reports (Sep 04 – early 05)
• Future Tasks– F. Countermeasure Trade-off Analysis
• Deliverables: Sensor Fusion Report (Oct 04), Report to Compare Countermeasures (early 05)
– G. Pilot FOT Planning• Deliverable: Final Report with FOT Plan (early 05)
![Page 13: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/13.jpg)
13
IDS Architecture
![Page 14: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/14.jpg)
14
StateMap-based Architecture
Other
Conflict predictor
State Map Generator
Gap predictorStop predictor
Safety applications
State map
Car GPS
Loops
Lidar/Radar
Sensors
Comm.
![Page 15: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/15.jpg)
15
Diverse IDS Implementations
Data Sources:
Information toDrivers:
Infrastructure Vehicle
LoopDetectors
VideoDetectors
Radar In-vehiclesensors
IDS Logic:-Analyze vehicle motions-Predict future gaps, conflicts or violations-Decide to issue alert
DynamicSign
SignalController
In-vehicledisplay
(wireless)
(wireless)
![Page 16: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/16.jpg)
16
DII
2070 Controller
Loops (11)EVT-300 (2) DensoLIDAR (2)
Wireless Modem
Serial Serial
Digital HI/LO
Implementation at TFHRC(1 of 3)
Fixed (Infrastructure) Sensors
Ethernet
QNX6 Computer
(Sensor Fusion, Warning, and Tracking Algorithms)
Serial
Controller Cabinets
![Page 17: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/17.jpg)
17
Implementation at TFHRC (2 of 2)
2070 contr.
Ethernet
QNX 6.0 computer(Sensor fusion, safety IDS algorithms)
Eth. card RS232
Sensors
I/O card
DII
Wireless 802.11aPCMCIA / PCI
card
infrastructure
vehicles
QNX 6.0 computer(State map visualizer)
Wireless 802.11aPCMCIA / PCI card
802.11a safety enhanced (more DSRC like)
![Page 18: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/18.jpg)
18
PATH RFS Test Intersection
Traffic Controller Cabinet
60 Meters of 3M Microloops
Embedded Loop Detectors
Signal Poles
RadarPoles
![Page 19: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/19.jpg)
19
Implementation at the PATH Intelligent Intersection
![Page 20: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/20.jpg)
20
Elements of Cooperative System at the PATH Intelligent Intersection
• 40-ft Bus at PATH/RFS– DVI at driver console,
with identical display for passengers
– Wireless implementation of simultaneous DII and DVI
• In practice, need not be simultaneous – a tricky research item
– Demonstrated to Barbara Sisson Associate Administrator for Research, FTA
![Page 21: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/21.jpg)
21
‘State Map’ of Intersection at ControllerWe know the status of every
communicating entity
SV Radar State
DII
Detected SV Speed (m/s)
SV Wireless Communication State
Detected POV D2I (m)
POV Radar State
Detected POV Speed (m/s)
POV Wireless Communication State
Total number of vehicles detected by SV and POV radars
In pavement loops
![Page 22: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/22.jpg)
22
Integrating the Driver IDS Alert System
![Page 23: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/23.jpg)
23
Perception - Difficulty Estimating Speed(Staplin, 1995)
10 50 100 150 200
30 mph
60 mph
132 m 4.96 s
80 m 3 s
184 m 6.91 s
158 m 5.93s
125 m 4.69 s
191 m 7.18 s
166 m 12.5s
104 m 7.8 s
228 m 17 s
161m 6s
108 m 4 s
214 m 8 s
100 m 7.5 s
51 m 3.8 s
149 m 11.2 s
150 m 11.7s
106 m 7.9 s
206 m 15.4 s
Age 20-53
Age 56-72
Age 75-91
![Page 24: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/24.jpg)
24
Observations and Experiments (1 of 3)
• Three Driver Experiments
1. Downtown Berkeley (ongoing)• Microscopic: 0.075-sec resolution
• Over 400 intersection traverses
• Defining key waypoints
2. Experimental Intersection: D2I vs T2I
3. Experimental Intersection: How well does our warning system / nominal DII work?
![Page 25: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/25.jpg)
25
Parameter Definitions
Crossing time
Turning time
Waiting time
![Page 26: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/26.jpg)
26
Observations and Experiments (2 of 3)
• Characterizing Driver Behavior at Intersections
– Four sites• Two in Berkeley• Pinole• Burlingame (near SFO)• San Francisco
– Two approaches• Roadside • In-Vehicle
– At least three warning algorithm approaches
• Dynamic (Shladover)• “BMI-based”/Statistical
(Ragland)• Neural net (Misener)
– Transcends LTAP/OD
![Page 27: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/27.jpg)
27
Observations and Experiments (3 of 3)Driver Infrastructure Interface (DII)
• Depth: “Looming Circle” DII further investigated– Our nominal DII
• Breadth: Based on CTCDC preparations and recommendations from committee, alternate DIIs to be considered
Looming Circle Looming Circle Looming Triangle Growing Arrow Looming Car
![Page 28: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/28.jpg)
28
Instrumented Ford Taurus for Driving Data Collection
• Wireless connection from laptop to PC104 in traffic signal control cabinet
• Intended Field data collection and controlled tests at RFS
![Page 29: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/29.jpg)
29
Output: Inputs to Alert System Design• Roadside observations (by radar and video):
– SV turning times– POV speed distribution– Accepted and rejected turning gaps– SV/POV speed change interactions– Behavior changes based on signal phase– Interactions with pedestrians
• Instrumented vehicle field tests:– Detailed SV speed-based information– Timing of turning decision
• Waypoints within turning movement (e.g., decision point, waiting time turning time)
– Influence of driver age and gender on turning behavior
![Page 30: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/30.jpg)
30
IDS Alert System Design
![Page 31: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/31.jpg)
31
Central Role of Alert System Design
Design IDS Alert System
Characterize Intersection Driving• Roadside Observations• Driver Observations
Evaluate COTS Technologies
Develop Simulation Tool
Develop Wireless CommunicationSystem
Develop DII
Driving ExperimentsAt RFS Intersection
Implement RFSTest Intersection
![Page 32: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/32.jpg)
32
Alert System Requirements
• Very low false negatives – consistently alert to hazardous conditions
• Low false positives – avoid alerting under benign conditions
• Appropriate timing of alert onset• Do as well as possible with imperfect sensor
data• …while recognizing inherent limitations:
– Subjective judgments of gap suitability– Diversity of driving population preferences
![Page 33: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/33.jpg)
33
Single LTAP/OD Encounter
SV
“Leading Buffer time” after POV1, ahead of SV
“Trailing Buffer time” after SV, ahead of POV2
POV1POV2SV
![Page 34: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/34.jpg)
34
LTAP/OD Alert Design Goals
• When an SV may be planning to make a left turn (in LT pocket or left lane, if no pocket):– Provide alert for gaps in POV traffic shorter than
minimum “safe” threshold– Don’t alert if POV gap is longer than acceptable
threshold (avoid nuisance alerts)– Time the alert to be early enough for SV driver to
be able to make a comfortable “no turn” decision, but not so early that it could be perceived a nuisance.
• First define alert design assuming complete and accurate sensor data, then adjust for imperfect data
![Page 35: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/35.jpg)
35
Nominal Alert Triggering Approach
• Develop for middle of green phase, then adjust for other conditions
• Anticipate when SV will enter dangerous region (in path of POVs), and how long it will need to clear that region
• Define acceptable time gap in approaching POV traffic after SV will clear intersection (“Trailing Buffer” time values based on field measurements and driver experiments, initial estimate ~ 3 s)
• Apply hysteresis in acceptable gap times to avoid chatter – Tgaplow and Tgaphigh, initially separated by 1 s (final value to depend on sensor and data fusion capabilities).
![Page 36: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/36.jpg)
36
Alert Development Process
• Define alert criteria, considering “all” expected combinations of conditions– Based on information from observation studies
(roadside and in-vehicle)– Adjustments for diverse intersection conditions
• Test alert criteria in simulation under challenging conditions, then adjust as necessary– POV speed changes (dilemma zone decel. and
accel., decel. to turn)– Range of approach speeds– Range of relative arrival times of SV, POV
• Test sensor alternatives to compare to “perfect” sensor information
![Page 37: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/37.jpg)
37
Sensor Coverage Alternatives
Standard loops at stop bar
Alt. 1
Single loop detector Double loop detector
Alt. 2
Radar coverage zone
Alt. 3
Alt. 4
Alt. 5
Double loop detector
Alt. 6
![Page 38: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/38.jpg)
38
General Findings on Alert Design and Detection Needs
• Complex interactions between SV and POV speeds need to be evaluated in multiple cases
• Early enough detection of POV is essential (6 seconds before stop bar)
• Use of final four loops at SV stop bar is crucial for determining SV speed in difficult cases
• Alerts depend on accurate speed as well as presence information
• POV speed changes close to intersection are not as important as SV speed changes
![Page 39: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/39.jpg)
39
Importance of Accurate Speed Measurements
• If assumed speed is below true speed, alerts will be late or missed completely
• The larger the speed difference (faster vehicles), the higher the likelihood of a problem– Also the most important cases for issuing
alerts (driver misperception of closing speed and severe consequences of crash)
• If assumed speed is above true speed, false alarms become more likely
• Variance of distribution of approaching speeds governs the importance of this
![Page 40: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/40.jpg)
40
Alert System Design Challenges
• Address complexity of SV-POV interactions• Accommodate diversity of driver preferences
within one set of alert criteria• Customize to fit differing requirements of
diverse intersections• Implement with affordable sensor suite
– Tracking individual vehicle positions and speeds, with multiple POVs
– Covering center of intersection as well as approach legs
![Page 41: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/41.jpg)
41
InfrastructureVehicle
IDS
Infrastructure based sensors
Traffic sign. contr.
Traffic sign. manag.
Traffic Signal
Inputs •Infrastructure state information (e.g. intersection type, signal state, etc);• Vehicles state information (e.g. speed, position, etc);
DII manager
Drivers
Signal controller
OutputDriver Interfaces
State Map Generator
Predictors
Future State Predictor
State map
Future State
Gap predictor Stop predictor Conflict predictor
In-vehicle sensors
In-vehicle display devices
![Page 42: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/42.jpg)
42
Cooperative System Opportunities
• Data from vehicles to intersection to improve IDS operation:– Longer-range (earlier) information– Acceleration to improve threat estimate– More accurate approach speed information– Enhanced weather or road surface status
• Data from intersection to vehicles:– IDS alert status for in-vehicle display– Information about approaching vehicle
threats not visible to vehicle sensors– Enhanced weather or road surface status
![Page 43: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/43.jpg)
44
Cooperative System Development Activities
• Define message sets needed for IDS applications, leaving flexibility for alternative implementations– Infrastructure-centered intelligence– Vehicle-centered intelligence– Lane – level accuracy maps
• Communication protocol design • Test wireless communication of these
message sets using 802.11a and DSRC test kits
• Fusion of intersection and vehicle sensor data
![Page 44: 1 Intersection Decision Support: A Cooperative Approach Summary of California PATH / UC Berkeley IDS Approach](https://reader036.vdocuments.us/reader036/viewer/2022062500/56649e2c5503460f94b1ac0c/html5/thumbnails/44.jpg)
45
Communication Needs Summary
• Vehicle broadcast message set per current SAE work, 42 byte payload and 70 byte overhead
• Assume (order of magnitude) 100 ms update interval
• Range of 300 m for higher-speed rural and 150 m for lower-speed urban environments
• Jam density could approach 600 vehicles within intersection range
• Accommodating 600 vehicles within DSRC safety channel (3 -- 27 Mbps) requires coordination in MAC protocol