I would use Linear Referencing to solve this problem. After making each road into a route with measures, I would use the Locate Features Along Route tool for both the traffic volume and the animal sightings to create two separate point event tables. Make sure the animal and volumes each have a unique ID field other than the ObjectID. Also make sure the two tables use different field names for the measure parameter of the tool so that when they combine both with be preserved. This will establish both sets of data relative to an identical road based measuring system and a common route reference that will be respected by the Linear Referencing tools (so an event does not jump from one road to another). Since you have to use a radius to tie the offset points to the routes, use the match all routes option and not the closest route option. Just throw out any routes that do not match the route associated with each sighting and volume (hopefully you can use a road name field associated with each data source to find the correct road to tie each event to).
Next I would get each sighting tied to all of the volumes on the road. This can be done by calculating the beginning and ending measure on each route (Python field calculation using either !Shape.FirstPoint.M! or !Shape.LastPoint.M!), joining that to the animal sightings and copying the table to a new table. Now that each animal is associated to the measures of the full limit of its route and will ignore all other routes, you can use the Overlay Route Events tool to combine this animal table with the volume table matching the full route measures to the volume measure and outputting a point event while preserving all fields. Because the AnimalSighting measure will be combined with the Volume measure now, a Separation field that holds the distance as measured along the road between the animal sighting and each volume point can be calculated using the VB Script formula Abs(Animal_Measure - Volume_Measure).
Then run the Sort tool on the combined event table using the RouteID, AnimalSightingID and the Separation fields as the sort criteria. Run the Summary Statistics tool in the Analysis toolbox, Statistics toolset to get the Min of the Object ID associate with each Route and AnimalSightingID value as the 2 case fields. Run my tool that creates a single key from a multi field key. Join on the new single field key and select SortedTable.ObjectID = SummaryTable.Min_ObjectID if you want the closest volume to the animal sighting or use SortedTable.ObjectID > SummaryTable.Min_ObjectID + 5 and SortedTable.AnimalSightingID = SummaryTable.AnimalSightingID if you want the top 5 closest volumes on that route. Export the selection and you have an event table of all animal sightings combined with the closest volumes you chose, plus you will know the distance separating them along the route.
You will be able to create several unique event layers by using different measures gathered at each step. For example you can create line segments between the animal sighting and each volume point or a point for either the animal or the volume all using this one table. If you preserved a side offset distance from the Locate Features Along Route tool used for the animal at the beginning you can offset the points or the line to the side of the line the animal was seen on using that field in the Make Route Event layer tool.
All of the tools above work under a Basic license.
