Reduce GPS jitters

my application is based on GPS data and for that I'm using Fused-location-provider. From now and then I have seen that there's a gps jitters and some GPS coordinates are off the road. This is unacceptable. What I tried to do was to implement Kalman filter and I did:

fun process(
    newSpeed: Float,
    newLatitude: Double,
    newLongitude: Double,
    newTimeStamp: Long,
    newAccuracy: Float
) {

    if (variance < 0) { // if variance < 0, object is unitialised, so initialise with current values
        setState(newLatitude, newLongitude, newTimeStamp, newAccuracy)
    } else { // else apply Kalman filter
        val duration = newTimeStamp - timeStamp
        if (duration > 0) { // time has moved on, so the uncertainty in the current position increases
            variance += duration * newSpeed * newSpeed / 1000
            timeStamp = newTimeStamp
        }

        val k = variance / (variance + newAccuracy * newAccuracy)

        latitude += k * (newLatitude - latitude)
        longitude += k * (newLongitude - longitude)
        variance *= (1 - k)

        retrieveLocation(newSpeed, longitude, latitude, duration, newAccuracy)
    }
}

And I'm using it whenever I receive new location as

KalmanFilter.process(
    it.newSpeed,
    it.newLatitude,
    it.newLongitude,
    it.newTimeStamp,
    it.newAccuracy
)

This helped to receive little bit more accurate results, but still - it did not fix the GPS jitter that was outside the road (see image):

Things I want to know:

1. Is my kalman filter implemented correctly? Is there way to improve current solution?
2. How to avoid a scenerios where coordinate is outside the road? (Something similar to snap-to-roads functionality, but as an algorithm inside app)

from Reduce GPS jitters