23 Acceleration Mode
The applicable ‘Acceleration Mode’ for a vehicle is determined at every time step. The assigned acceleration mode informs the specified/calculated acceleration rate.
23.1 UnimpededNormal
In this acceleration mode, the vehicle motion is not affected by a leader vehicle or by any traffic controls. The acceleration rate that gets applied is a function of the vehicle’s current velocity (v), as follows:
- v < desired speed, assign desired acceleration (e.g., a vehicle is discharging from a queue after a signalized control point turns from a red to green indication.
- v > desired speed, assign minimum deceleration; Default value = \(-5\) ft/s/s. This value can be changed in the Settings, under the ‘General Vehicle Movement’ section.
Note that there is a small tolerance threshold for the desired speed; that is, a vehicle is considered to be traveling at its desired speed if it is within the following range:
\[\biggl\lbrack DesiredSpeed\times\frac{100-DesiredSpeedDeviation}{100}\biggl\rbrack\leq DesiredSpeed\leq \biggl\lbrack DesiredSpeed\times\frac{100+DesiredSpeedDeviation}{100}\biggl\rbrack\]
where:
\({DesiredSpeed}=\) desired speed of subject vehicle, in ft/s. The determination of this value is described here, and
\({DesiredSpeedDeviation}=\) desired speed margin, in ft/s. This value is a function of the Driver Type.
23.2 CarFollow
If a vehicle meets the following conditions:
- is in a following status (Leader/Follower Status)
- is not stopped (i.e., velocity > 0)
- is not stopping for a control
then an acceleration rate will be calculated according to the car-following model.
23.3 StoppingForQueue
This acceleration mode applies when a vehicle needs to decelerate to join the back of a queue. The applied deceleration rate is calculated as follows:
\[DecelRateStopForQueue=-0.5\times\frac{VehicleSpeed^2}{DistanceToLeadVehicle-StoppingBuffer}\]
where:
\({DecelRateStopForQueue}=\) deceleration rate required to bring vehicle to a stop behind its lead vehicle (the last vehicle in queue), ft/s/s,
\({DistanceToLeadVehicle}=\) distance between front bumper of subject vehicle and rear bumper of lead vehicle, in ft, and
\({StoppingBuffer}=\) Stop gap for vehicle fleet type, in feet (values can be changed in the Settings, under the ‘Car Following’ section).
23.4 StoppingForControl
This acceleration mode applies when a vehicle needs to decelerate to stop immediately upstream of a control point, such as for a signalized intersection. The applied deceleration rate is calculated as follows:
\(DecelRateStopForControl=-0.5\times\frac{VehicleSpeed^2}{DistanceToControlPoint-StoppingBuffer}\)
where:
\({DecelRateStopForControl}=\) deceleration rate required to bring vehicle to a stop behind the control point, ft/s/s,
\({DistanceToControlPoint}=\) distance between front bumper of subject vehicle and control point location, in ft, and
\({StoppingBuffer}=\) 2 ft (values can be changed in the Settings, under the Car Following section).
23.5 Stopped
This acceleration mode applies any time a vehicle is stopped (velocity = 0). This typically occurs when a vehicle is stopped due to a control point (e.g., signalized intersection, ramp meter), either within the queue or at the front of the queue.
23.6 MaxAccel
Regardless of the acceleration mode assigned to a vehicle, the maximum acceleration of a vehicle always serves as a constraint against which other candidate acceleration values are checked. The maximum acceleration value is not static and is not user-definable. Rather, it is calculated internally as a function of numerous factors, as described here.
If another acceleration mode is initially assigned and its resultant acceleration value exceeds that of the calculated maximum acceleration value for the vehicle, the acceleration mode will be reassigned to ‘MaxAccel’. The maximum acceleration mode is typically applied in situations where a vehicle, usually a commercial truck, is attempting to maintain its desired speed on an upgrade.
23.7 Max Decel
The default value is \(-19\) ft/s/s. This value is consistent with what is generally considered to be an emergency braking deceleration rate. This value can be changed in the Settings, under the ‘General Vehicle Movement’ section.
23.8 Desired Accel
This value is calculated as:
\[DesiredAcce{l_i}=BaseDesiredAccelRate\times DesiredAccelRateMultiplie{r_i}\]
where:
\({DesiredAccel_i}=\) desired acceleration rate for driver type i, in ft/s/s,
\({BaseDesiredAccelRate}=\) base desired deceleration rate for all driver types, in ft/s/s. Default value = 4 ft/s/s (this value can be changed in in the Settings, under the ‘General Vehicle Movement’ section), and
\({DesiredAccelRateMultiplier_i}=\) acceleration rate multiplier applied to desired acceleration rate for driver type i (proportion). This value can be changed in the Settings, under the ‘Drivers’ section.
23.9 Desired Decel
\[DesiredDece{l_i}=BaseDesiredDecelRate\times DesiredDecelRateMultiplie{r_i}\]
where:
\({DesiredDecel_i}=\) desired deceleration rate for driver type i, in ft/s/s.
\({BaseDesiredDecelRate}=\) base desired deceleration rate for all driver types, in ft/s/s. Default value = \(-10\) ft/s/s. This value can be changed in the Settings, under the ‘General Vehicle Movement’ section).
\({DesiredDecelRateMultiplier_i}=\) deceleration rate multiplier applied to desired deceleration rate for driver type i (proportion). This value can be changed in the Settings, under the ‘Drivers’ section.
23.10 DownstreamLinkSpeedChange
For situations in which the assigned free-flow speed (FFS) for a vehicle’s destination link is different than the FFS for its current link, the following rules are applied:
- Destination Link FFS > Current Link FFS: Vehicle will attempt to accelerate to achieve higher desired speed (as a function of higher link FFS), starting when it enters destination link
- Destination Link FFS < Current Link FFS: Vehicle will attempt to decelerate to achieve lower desired speed (as a function of lower link FFS), starting upstream of destination link entry location.
More specifically:
\[DecelDistance=\frac{VehicleSpeed^2 - DesiredSpeedDestinationLink^2}{2\times BaseDecelRate}\]
where:
\({DecelDistance}=\) distance required to slow vehicle to its desired speed for the destination link, ft,
\({VehicleSpeed}=\) distance between front bumper of subject vehicle and rear bumper of lead vehicle, in ft,
\({DesiredSpeedDestinationLink}=\) desired speed for subject vehicle, based on the assigned free-flow speed of the destination link, and
\({BaseDecelRate}=\) desired deceleration rate / 4.
If the DistanceToDestinationLink \(\leq\) DecelDistance, calculate
\[DecelRateForLinkSpeedChange=-0.5\times\frac{VehicleSpeed^2 - DesiredSpeedDestinationLink^2}{DistanceToDestinationLink}\]
Use the minimum of: BaseDecelRate, DecelRateForLinkSpeedChange, MaxDecel
where:
\({DecelRateForLinkSpeedChange}=\) deceleration rate required to bring vehicle to its desired speed for the destination link, ft/s/s,
\({DistanceToLeadVehicle}=\) distance between front bumper of subject vehicle and rear bumper of lead vehicle, in ft, and
\({DistanceToDestinationLink}=\) difference between position of vehicle on current link and the start location of its destination link, ft.