Acceleration Mode

Main Page > Technical Documentation > Vehicle Movement Logic > 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.

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 'VehicleMovementParms.xml' file.

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

CarFollow

If a vehicle meets the following conditions:
 * is in a following status (discussed here)
 * is not stopped (velocity > 0)
 * is not stopping for a control

then an acceleration rate will be calculated according to the car-following model (discussed here).

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

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

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.

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 discussed in detail 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.

MaxDecel

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 'VehicleMovementParms.xml' file.

DesiredAccel

This value is calculated as:

$$DesiredAcce{l_i}=BaseDesiredAccelRate\times DesiredAccelRateMultiplie{r_i}$$

where

DesiredDecel

$$DesiredDece{l_i}=BaseDesiredDecelRate\times DesiredDecelRateMultiplie{r_i}$$

where

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

If the DistanceToDestinationLink $$\leq$$ DecelDistance, calculate $$DecelRateForLinkSpeedChange=-0.5\times\frac{VehicleSpeed^2 - DesiredSpeedDestinationLink^2}{DistanceToDestinationLink}$$ Use the minimum of: BaseDecelRate, DecelRateForLinkSpeedChange, MaxDecel

where

SlowingForDowngrade

Forthcoming