CORSIM networks contain no vehicles at the beginning of a run. As the first seconds are simulated, vehicles are emitted onto the network from entry and source nodes. The time required to fill the network with traffic is referred to as the initialization period. Since the initialization period does not accurately represent the conditions to be modeled, no statistics are gathered during this period. A check is made at the end of every time interval for equilibrium, i.e. the end of initialization. Equilibrium is assumed when the number of vehicles in the network is within 8% of the number of vehicles in the network during the previous time interval, and within 12% of the number of vehicles in the network during the second previous time interval. In the CORSIM output file, this information is reported in the section called “Initialization Statistics”.
This is usually caused by the lane drop warning sign distance. The warning sign for a lane drop should never be at a location with only one lane. The warning sign location is really the point at which vehicles respond to the object associated with the warning sign. If there is only one lane at the point, or only one lane usable by certain vehicles, the warning sign will be ignored.
If you are using “flow rate variations within a time period” for a given link, then the warning message is nothing to worry about for that particular link. If you are using flow rate variations within a time period for a given link, then the flow rate will not be zero for that particular link.
Fatal error 6500 is usually related to incorrect specification of link type, i.e. ramp versus mainline. The basic rule is that there has to be a continuous path of mainline links from an exit or exit interface node to an entry or entry interface node.
The message about Missed Destinations states that a vehicle was unable to get to its freeway off-ramp and missed its destination. It will be rerouted to the next off-ramp downstream. This will change the percentages of vehicles exiting at those off-ramps. You should try to determine why so many vehicles are missing their destinations. Consider off-ramp warning sign locations, and any messages from CORSIM about vehicles not having acceptable candidate lanes to travel in. The other message (“Computed leader…”) indicates that the FRESIM lane change logic broke down, possibly because of incorrectly specified connections between freeways.
This might indicate that the bus station numbers are out of sequence on record type 188. Apparently the station number sequence (on record type 188) needs to be consistent with the order in which stations would be encountered by a bus driver.
Fatal error messages 6197 and 6198 are often caused by a missing record type 25. Off-ramps are defined by record type 25. For the mainline link, record type 25 must be coded to indicate which node receives off-ramp traffic. Without record type 25, CORSIM cannot locate the off-ramp destination node. When all of the necessary record type 25′s are entered, this allows the origin-destination inputs on record type 74 to function properly.
There is a software queue for each entry node. Vehicles are put into the queues based on the volume inputs and are ordered by their assigned time of entry. When the simulation reaches the entry time of the vehicle at the front of the queue that vehicle will enter the network. If it is unable to enter the network it remains the first vehicle in that queue, and it will try to enter on every successive time step until it is able to. No vehicles are lost, but some may still be in queue when the simulation ends. If so, volume would be reduced by the number of vehicles still in queue.
FRESIM was unable to generate a balanced O-D table using your inputs. The CORSIM Reference Manual discusses that issue in the Overview for RT 74. You may need to adjust your inputs as described in that overview to get a better balanced table.
Some possible causes include:
- The default off-ramp warning sign distance (2500′) extends beyond the boundaries of the FRESIM network, and must be shortened.
- A combination of high travel speeds and short deceleration lane length makes it difficult for vehicles to switch into the deceleration lane.
- CORSIM can get confused when multiple geometric objects are located at exactly the same location. For example, a lane drop located directly on top of a node can cause CORSIM to have trouble maintaining leader follower chains in that area. Once a chain gets corrupted it can cause a wide variety of errors, including vehicles in non-existent lanes (fatal error 6710) and logic getting stuck in infinite loops (simulation not finishing). Moving the lane drop 1 foot away from the upstream node may allow the simulation to run without error.
- There are too many anticipatory lane changes, and eliminating them can eliminate the warning messages. Anticipatory lane changes can be eliminated by right clicking on the appropriate freeway link in TRAFED, selecting the “Lanes” tab, and then setting the anticipatory lane change parameters to 1 mph and 1 foot, respectively.
- Off-ramp warning signs are not located where at least one lane leads to the off-ramp, or warning signs are located where they would reduce the number of usable lanes to zero. For example, if a vehicle crosses an off-ramp warning sign indicating that the only acceptable lane is lane 1, and then it crosses a lane drop warning sign that indicates that lane 1 cannot be used, there will be no lanes for that vehicle to use. If that occurs, the vehicle will ignore what the warning signs have indicated and just stay in the current lane. In that case the off-ramp warning sign should have been placed downstream from the lane drop. There are many variations of that concept with different types of geometries.
There are multiple possibilities for the leader-follower error:
There is a known geometry that can cause a leader-follower error. The geometry involves a cloverleaf where a vehicle in searching for a leader can follow a path that leads to itself as its leader. There is a work around that involves breaking the path so that a vehicle will not find itself as its leader. One way to accomplish this is to break the loop by adding a small NETSIM link in one of the loops of the cloverleaf. Another solution is to break one of the full auxiliary lanes between connectors into an acceleration lane followed by a deceleration lane with at least on foot separation between the two. The NETSIM link solution is probably the cleanest of the two because you can maintain the cloverleaf geometry better. This problem is more likely to occur with networks that have very few vehicles but can occur in any network that has a continuous cloverleaf.
Another known geometry that can cause a leader-follower error involves a freeway that branches (diverges) and then branches (merges) back onto itself. The result is that there are two separate paths from the downstream end of the freeway to the upstream end. That causes inconsistencies in locations calculated for vehicles on those two paths. If the paths have different lengths, a vehicle may appear to be downstream of the subject vehicle when in reality it is upstream. When the two vehicles merge onto the same path, the follower is ahead of the leader. Whenever a freeway branches and then branches back onto itself the two paths must be broken into two segments; which can be done by adding dummy exit and entry nodes, or by inserting a NETSIM section into one of the ramps.
Short link lengths can cause a corruption in the leader-follower chain. When the link is short enough, and the free flow speed is high enough, it becomes possible for a vehicle to completely jump over the short link during a one-second time step. When this happens, CORSIM’s vehicle processing logic completely breaks down, and the result is usually leader follower errors. CORSIM sends red-colored warning messages to the TSIS output window, when it detects links that are short enough to cause modeling problems.
Although the most common cause of problems with the chain is circular geometries, the second most common would be freeways that split into two branches and then rejoin downstream. Vehicle positions, which are measured from the end of the freeway, can be inconsistently calculated if there are two separate paths to get to the end of the freeway. Incorrect positions can cause errors in determining leaders and followers. Once a branch splits off from a freeway it should go to an exit node and not go back onto the freeway that it branched off of. NETSIM links may be required where the branch rejoins the freeway.
CORSIM can get confused when multiple geometric objects are located at exactly the same location. For example, a lane drop located directly on top of a node can cause CORSIM to have trouble maintaining leader follower chains in that area. Once a chain gets corrupted it can cause a wide variety of errors, including vehicles in non-existent lanes (fatal error 6710) and logic getting stuck in infinite loops (simulation not finishing). Moving the lane drop 1 foot away from the upstream node may allow the simulation to run without error.
Correcting link free flow speeds can solve the problem. FRESIM links connected to an interface node should have the same free flow speed as NETSIM links on the opposite side.
A problem with the leader follower chain can be caused by a 2.0 second lane change interval and lane drop that has a warning sign only 1 foot upstream from the lane drop. The warning sign location should always be far enough upstream so that vehicles can stop before hitting the lane drop if they can’t make a lane change. Vehicles hit the end of the lane and then other vehicles behind them pile up onto them. Somewhere in that pile up the leader follower chain gets corrupted.
