Suggested Paramics Settings
From Traffic Analysis and Microsimulation
This section contains suggested calibration parameter settings based on Wisconsin experience with Quadstone Paramics. The use of these suggestions may help achieve a well-functioning model more quickly, but do not guarantee success.
Mean Target Headway
By definition, headway is the time between two successive vehicles as they pass a point on the roadway, measured from the same common feature of both vehicles (for example, the front axle or the front bumper). As vehicles drive closer together the headway decreases and the throughput of the roadway increases. A related concept is the gap, which is the time or distance from the back bumper of the first vehicle to the front bumper of the second vehicle.
Paramics is a "gap acceptance model" which means that several vehicle behaviors depend on the minimum headway vehicles are wililng to accept. An example is the smallest gap that a vehicle will accept when it merges or changes lanes. In Paramics this behavior is very strongly influenced by the the MEAN TARGET HEADWAY coefficient.
In effect, reducing the MEAN TARGET HEADWAY coefficient makes vehicles drive more aggressively and increases capacity. Therefore, this coefficient needs to be adjusted based on the prevalent "driving style" in the area that is being modelled. People tend to drive more aggressively in big, congested cities than they do in uncongested rural areas or small towns, so the following settings are suggested:
- Urban areas: 0.85 to 0.90
- Small Cities: 0.90 to 0.95
- Rural areas: 0.95 to 1.00
Generalized Cost Coefficients
Most Paramics networks have situations where vehicles can use more than one route between a given origin and destination. In these situations Paramics uses a routing method similar to Wardrop Equilibrium. The route choice algorithm is strongly influenced by the GENERALIZED COST COEFFICIENTS that are set in the CORE NETWORK ATTRIBUTES --> CONFIGURATION --> ADVANCED --> ASSIGNMENT tab. These coefficients determine how much weight to assign to time, distance, and toll price.
Consider the case of a driver who is on westbound I-94 heading toward the Madison area, and wants to get to the WisDOT headquarters at Hill Farms. Somewhere east of the Badger Interchange, that driver will need to decide whether to go straight through downtown Madison, or take the South Beltline which skirts the south side of the city. The route through downtown is several miles shorter, but the Beltline is usually a bit faster.
By default, Paramics assumes that such decisions are made 100% on the basis of time, but the coefficients can (and should) be adjusted to take distance and tolls into consideration. For example, with the default setting Paramics would assume that all drivers going from the Badger Interchange to Hill Farms use the quickest route, which is usually the Beltline (except when it is congested). But the time difference is usually small, and some drivers prefer the route through downtown since it seems more direct and they will probably burn less fuel due to the reduced distance.
Based on the relative values of time and fuel established in the Wisconsin Economic Analysis Guidelines, the following coeffients are suggested for routes without tolls:
- 0.667 Time
- 0.333 Distance
- 0.000 Toll Price
Suggested coeffiecients for routes with tolls have not been established for Wisconsin.
Please see this PDF file.
- HEAVIES USE ALL LANES: Checking this box in the CONFIGURATION --> ADVANCED --> OPTIONS menu improves merging behavior by spreading trucks out into all mainline lanes. Wisconsin does not require trucks to stay in the right lane, so this setting is recommended for Wisconsin models.
Coding of Major Signalized Intersections (Junctions)
There are many ways to code large signalized intersections in Paramics, but some methods result in problems with vehicle behavior such as failure to use all lanes properly. This coding method described has been found successful and is illustrated in <A HREF="http://www.wisdot.info/microsimulation/IntersectionCodingDemo.zip">this demo</A>.
- Code the intersection entirely as two-way links and signalize a single node representing the center of the intersection.
- Mange all lane offsets from the centerline using the MEDIAN WIDTH feature in the CATEGORIES file. There is no user interface for this--it must be set up using a text editor. For example, here are three related categories with offsets from the centerline of 0 feet, 12 feet, and 24 feet. (Note that the WIDTH includes the width of the median measured from the centerline).
category 101 lanes 2 speed 35 mph width 24.0 ft colour 0x0000ff00 type urban major median width 0.0 ft headway factor 1.000 curve speed factor 0.0 toll 0.000 cost factor 1.000 signpost 820.2 ft,3.3 ft category 111 lanes 2 speed 35 mph width 36.0 ft colour 0x0000ff00 type urban major median width 12.0 ft headway factor 1.000 curve speed factor 0.0 toll 0.000 cost factor 1.000 signpost 820.2 ft,3.3 ft category 121 lanes 2 speed 35 mph width 48.0 ft colour 0x0000ff00 type urban major median width 24.0 ft headway factor 1.000 curve speed factor 0.0 toll 0.000 cost factor 1.000 signpost 820.2 ft,3.3 ft
- Many intersections have extra lanes for left and right turns. These extra lanes must be added one at a time. For example, when widening from 3 lanes to 5 to create a double left turn lane, there must be a 4 lane section in between. The transitino section can be fairly short, but Paramics has rounting problems if it does not exist. (Note: these statements are based on Quadstone Paramics 6.9.0 and could change in later versions. "Left" and "right" refer to North American driving.)
- At intersections with a double left turn lane, the widen from 3 to 4 lanes is usually the start of the left turn bay (in Wisconsin and other places that drive on the right). To avoid unnatural vehicle behavior add NEXTLANES 1:1, 2:2 and 3:3 .
- Again using the example of a double left, the widen from 4 to 5 lanes is usually the start of the right turn bay. To avoid unnatural vehicle behavior add NEXTLANES 2:2, 3:3, 4:4, and 5:5 .
- Continuing the examuple of a double left, at the widen from 4 to 5 lanes, set the SIGNPOSTING to PROPOGATE .
- The use of extra nodes to create "pork chop" islands for free-flow right turn lanes is not recommended unless the right turn bypass volume is quite large or the visual realism of the pork chop island is essential for public outreach.
- Verify the NEXTLANES at the main node, e.g. if there is a double left make sure there are two receiving lanes.
- Verify the node positions for the 3 to 4 and 4 to 5 lane transitions, i.e. make sure the left and right turn bays are long enough to handle the actual volume.
- Verify the signal timing.