Intelligent Roadway Information System

Variable Speed Advisory

The [vsa] action tag is used to display speed advisory warnings on signs. It will cause a message to be displayed only when approaching congestion — otherwise the message will be blank. The tag itself will be replaced with an advisory speed, but the rest of the message can be any valid MULTI string.

Uniform Acceleration

The goal is to promote a safe, uniform deceleration for vehicles approaching congestion. Speed advisory signs are activated to warn motorists of slow traffic ahead.

A bottleneck is activated when the uniform rate of deceleration is found to be unsafe. The standard equation for uniform acceleration is used to determine when variable speed advisories must be deployed.

Policy Parameters

Each mainline station has an associated speed limit L (mph), as well as the distance from the upstream end of corridor to station. These are used to calculate advisories. There are also system attributes to control system-wide parameters.

System Attribute Default Description
vsa_bottleneck_id_mph 55 mph Maximum speed at a station to be identified as a bottleneck
vsa_control_threshold -1000 Threshold for control distance (mi/hr²)
vsa_downstream_miles 0.2 mi Largest distance downstream of a bottleneck to activate signs
vsa_max_display_mph 55 mph Maximum advisory speed to display
vsa_min_display_mph 30 mph Minimum advisory speed to display
vsa_min_station_miles 0.1 mi Minimum distance between stations
vsa_start_intervals 3 Intervals to activate a bottleneck
vsa_start_threshold -1500 Threshold to activate a bottleneck (mi/hr²)
vsa_stop_threshold -750 Threshold to deactivate a bottleneck (mi/hr²)

Station Sample Speeds

Speed in miles per hour, average of lanes in station. A rolling average is used, with the number of sample intervals depending on the current average station density. Samples older than the oldest sample used in the previous time step are not used. Also, the sample count is reduced to 2 whenever there is a clear upward or downward trend in the last 3 speed samples.

Current Density Sample Count Sample period
below 10 vpm N/A N/A (Speed Limit L)
10-15 vpm 6 samples 3 minutes
15-25 vpm 4 samples 2 minutes
25-40 vpm 3 samples 1.5 minutes
40-55 vpm 4 samples 2 minutes
above 55 vpm 6 samples 3 minutes

Deceleration Thresholds

NOTE: using precipitation rates to control thresholds is not yet implemented.

There are three thresholds for determining a bottleneck: start, control, and stop. The thresholds are based on the pavement conditions, measured using a precipitation sensor. There are seperate thresholds for dry, light rain, moderate rain, heavy rain, and ice. The units are in mi/hr², and negative values indicate deceleration instead of acceleration.

Precipitation Start Control Stop
Dry -1500 -1000 -750
Light rain -1200 -800 -600
Moderate rain -1050 -700 -525
Heavy rain -900 -600 -450
Ice -750 -500 -375