Is linear regression fit for this data
For the first question, it is important to recall that RMSE has the same unit as the dependent variable. It means that there is no absolute good or bad threshold, however you can define it based on your DV. For a datum which ranges from 0 to 1000, an RMSE of 0.7 is small, but if the range goes from 0 to 1, it is not small.

I would do some feature engineering (Create more variables: Time of Day, Day of Week, Month etc...) and run it through a Neural Network and then check for accuracy.  You may want to check if there is any correlation between the 4 junctions as well but if you run a NN, you don't have to.