I strongly suggest you hire an acoustical expert firm to perform such
calculation. The main reason is that there is a lot of parameters to
consider when preparing a road traffic noise .
But, if you still want to purchase a ling software, you might be
interested in TNM 2.5 (Traffic Noise ) available at
http://mctrans.ce.ufl.edu/featured/trafficNoise/ which is a lot
cheaper than either Cadna or Soundplan.
For your information, we are users of Cadna and TNM.
Best regards.
Jeff Latour
MJM Acoustical Consultants Inc.
www.mjm.qc.ca